First pass at a language reference (#1279)

* First pass at a language reference

We already had the `language-guide.md` document under `docs/`, but this is an attempt to introduce a more full-featured reference to the Slang language and its features.

Right now it is mostly focused on the syntax and what the language allows to be declared, and it is a little light on semantic details throughout (mostly relying on familiarity with C to explain the things that are left unsaid). Even so, this hopefully provides a starting point to continue adding more detail.

* typos and other small fixes

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+Lexical Structure
+=================
+
+Source Units
+------------
+
+A _source unit_ comprises a sequence of zero or more _characters_ which for purposes of this document are defined as Unicode scalars (code points).
+Implementations *may* accept source units stored as files on disk, buffers in memory, or any appropriate implementation-specified means.
+
+Encoding
+--------
+
+When encoding is required, source units *should* be encoded using UTF-8.
+Implementations *may* support additional implemented-specified encodings.
+
+Whitespace
+----------
+
+_Horizontal whitespace_ consists of space (U+0020) and horizontal tab (U+0009).
+
+A _line break_ consists of a line feed (U+000A), carriage return (U+000D) or a carriage return followed by a line feed (U+000D, U+000A).
+Line breaks are used as line separators rather than terminators; it is not necessary for a source unit to end with a line break.
+
+Escaped Line Breaks
+-------------------
+
+An _escaped line break_ comprises a backslack (`\`, U+005C) follow immediately by a line break.
+
+Comments
+--------
+
+A _comment_ is either a line comment or a block comment:
+
+```hlsl
+// a line comment
+/* a block comment */
+```
+
+A _line comment_ comprises two forward slashes (`/`, U+002F) followed by zero or more characters that do not contain a line break.
+A line comment extends up to, but does not include, a subsequent line break or the end of the source unit.
+
+A _block comment_ begins with a forward slash (`/`, U+002F) followed by an asterisk (`*`, U+0052). 
+A block comment is terminated by the next instance of an asterisk followed by a forward slash (`*/`).
+A block comment contains all characters between where it begins and where it terminates, including any line breaks.
+Block comments do not nest.
+It is an error if a block comment that begins in a source unit is not terminated in that source unit.
+
+Phases
+------
+
+Compilation of a source unit proceeds _as if_ the following steps are executed in order:
+
+1. Line numbering (for subsequent diagnostic messages) is noted based on the locations of line breaks
+
+2. Escaped line breaks are eliminated. No new characters are inserted to replace them. Any new escaped line breaks introduced by this step are not eliminated.
+
+3. All comments are replaced with a single space (U+0020)
+
+4. The source unit is _lexed_ into a sequence of tokens according the lexical grammar in this chapter
+
+5. The lexed sequence of tokens is _preprocessed_ to produce a new sequence of tokens (Chapter 3)
+
+6. Subsequent processing is performed on the preprocessed sequence of tokens
+
+Identifiers
+-----------
+
+An _identifier_ begins with an uppercase or lowercase ASCII letter (`A` through `Z`, `a` through `z`), or an underscore (`_`).
+After the first character, ASCII digits (`0` through `9`) may also be used in an identifier.
+
+The identifier consistent of a single underscore (`_`) is reserved by the language and must not be used by programs.
+Otherwise, there are no fixed keywords or reserved words.
+Words that name a built-in language construct can also be used as user-defined identifiers and will shadow the built-in definitions in the scope of their definition.
+
+Literals
+--------
+
+### Integer Literals
+
+An _integer literal_ consists of an optional radix specifier followed by digits and an optional suffix.
+
+The _radix specifier_ may be:
+
+* `0x` or `0X` to specify a hexadecimal literal (radix 16)
+* `0b` or `0B` to specify a binary literal (radix 2)
+
+When no radix specifier is present a radix of 10 is used.
+
+Octal literals (radix 8) are not supported.
+A `0` prefix on an integer literal does *not* specify an octal literal as it does in C.
+Implementations *may* warn on integer literals with a `0` prefix in case users expect C behavior.
+
+The _digits_ of an integer literal may include ASCII `0` through `9`.
+In the case of a hexadecimal literal, digits may include the letters `A` through `F` (and `a` through `f`) which represent digit values of 10 through 15.
+It is an error for an integer literal to include a digit with a value greater than or equal to the radix.
+The digits of an integer literal may also include underscore (`_`) characters, which are ignored and have no semantic impact.
+
+The _suffix_ on an integer literal may be used to indicate the desired type of the literal:
+
+* A `u` suffix indicates the `uint` type
+* An `l` or `ll` suffix indicates the `int64_t` type
+* A `ul` or `ull` suffix indicates the `uint64_t` type
+
+### Floating-Point Literals
+
+> Note: This section is not yet complete.
+
+### String Literals
+
+> Note: This section is not yet complete.
+
+### Character Literals
+
+> Note: This section is not yet complete.
+
+Operators and Punctuation
+-------------------------
+
+> Note: This section is not yet complete.