Don't let clang-format reorder Fiddle `#include`s (#7887)

The documentation added by #6844 included instructions to make sure that
the Fiddle `#include` in a file comes after all the other `#include`s,
but it's easy to accidentally violate this via `clang-format`, as
happened for `source/slang/slang-ast-modifier.h` in #7559. This PR
guards against this sort of violation by separating all Fiddle
`#include`s from other `#include`s via a blank line followed by a `//`
line (as we already do in most cases), and also adds a sentence about
this in `tools/slang-fiddle/README.md`.

As a bonus, I also enabled Markdown syntax highlighting for all the code
blocks in that doc file.

Co-authored-by: Ellie Hermaszewska <ellieh@nvidia.com>

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