[Parser][NFC] Create a public wat-lexer.h header

[tlively]

wat-parser-internal.h was already quite large after implementing just the lexer,
so it made sense to rename it to be lexer-specific and start a new file for the
higher-level parser. Also make it a proper .cpp file and split the testable
interface out into wat-lexer.h.

[tlively]

Current dependencies on/for this PR:

• main
  • PR [Parser][NFC] Create a public wat-lexer.h header #4695 👈
    • PR [Parser][NFC] Remove extraneous braces from std::optional returns #4696
      • PR [Parser][NFC] Improve comments about default NaN payloads #4697

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[aheejin]

You said you named the previous header wat-parser-internal.h because this was not meant to be used from outside, and you were gonna create a new header with public API. Now these headers don't have internal in the names, are they meant to be public? But this has the same contents as the previous wat-parser-internal.h.

[aheejin]

Huh, I wonder why git doesn't recognize this as a file move wat-parser.cpp->wat-lexer.cpp..

[tlively]

Looks like it does now 👍

[tlively]

If there were a good way to make a hierarchy of headers, then I would make wat-lexer.h a private header within the parser module so that it could be used from a future wat-parser.cpp and from nowhere else. Unfortunately there isn't a great way to do that in C++. If we're going to split the lexer's interface declarations from its implementation (which I think makes sense given how large the implementation is), then I don't see any benefit in marking the header "internal" anymore.

I doubt any other code in binaryen will want to use the lexer directly, but if it does, that would be ok after this change.

[tlively]

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