[Parser] Simplify the lexer interface

[tlively]

The lexer was previously an iterator over tokens, but that expressivity is not actually used in the parser. Instead, we have input.h that adapts the token iterator interface into an iterface that is actually useful.

As a first step toward simplifying the lexer implementation to no longer be an iterator over tokens, update its interface by moving the adaptation from input.h to the lexer itself. This requires extensive changes to the lexer unit tests, which will not have to change further when we actually simplify the lexer implementation.

[tlively]

Current dependencies on/for this PR:

• #6319 👈
• main

This stack of pull requests is managed by Graphite.

[tlively]

Going to go ahead and land this despite the unrelated emscripten failure.