Add a `-o` option to command-line `slangc`

Fixes #11

- This adds a `-o` command-line option for specifying an output file.

- The code tries to be a bit smart, to glean an output format from a file extension, and also to associate multiple `-o` options with multiple `-entry` options if needed.

- There is a restriction that all the output files need to agree on the code generation target. This is reasonable for now, but might be something to lift eventualy

- There is a restriction that only one output file is allowed per entry point
  - Together with the previous item this means you can't output both a `.spv` and a `.spv.asm` in one pass, even though both should be possible

- There is currently a restriction that output paths only apply to entry points
  - This means there is no way to output reflection JSON to a file with `-o` (but that is mostly just a debugging feature for now)
  - This also means we don't support any "container" formats that can encapsulate multiple compiled entry points

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