[wgsl-in] Implement template list discovery and handle all type resolution in the lowerer

[teoxoy]

Connections
Resolves #4501.
Resolves #4406.
Resolves #4405.

Description
Almost all commits prior to implement template list discovery are refactors to get the frontend to look more similar to the spec and to make the needed changes easier to implement (most notably making the Tokenizer no longer impl Clone).

The template list discovery implementation differs from the spec only in structure (not in behavior) since we can do it lazily after tokenization.

The last 3 substantial commits move all type resolution to the lowerer since all identifiers that are part of types need to be resolved the same as all other identifiers.

Testing
Changed existing tests and added new ones.

Squash or Rebase?
Rebase.

[jimblandy]

I can take review here - I've been looking into this for a long time.

[jimblandy]

@teoxoy It seems like a bunch of these early commits would be fine as independent cleanup PRs, like 42338ff and 4adede5.

[jimblandy]

I think lhs_expression is spec jargon. Isn't there something we could say here that would be more readily recognized by WGSL programmers? "expression to be assigned to"? "destination for assignment"?

(Spec jargon is fine when we need to be precise and there's nothing better.)

[teoxoy]

"destination for assignment"/"assignment destination" sound good.
It's also used by increment/decrement statements but it's probably ok?

[jimblandy]

So far I've reviewed the following commits and they all look fine:

527cf31a1 | * rename `function_call_or_assignment_statement` to `func_call_or_variable_updating_statement`
16c2777ba | * rename `function_statement` to `func_call_statement`
086c0e4c3 | * remove `lexer.clone()`
fe63d0753 | * rename `postfix` to `component_or_swizzle_specifier`
47108f674 | * update `lhs_expression` to be in line with the spec
75fbca1ed | * remove `peek_ident_with_span`
ed6a03205 | * remove `next_scalar_generic`
e99eb0149 | * rename `general_expression` to `expression`
54fe3ca2e | * rename `assignment_statement` to `variable_updating_statement`
4adede577 | * [wgsl] rename `type_decl` to `type_specifier`
42338fff3 | * add `optionally_typed_ident` fn

[teoxoy]

I removed the last commit that was adding a CTS test since it wasn't passing due to unrelated reasons and pushed 2 more commits to fix the issues CI surfaced.

[teoxoy]

@teoxoy It seems like a bunch of these early commits would be fine as independent cleanup PRs, like 42338ff and 4adede5.

They could but that would be lots of separate PRs to file and orchestrate since I think some of them depend on others. I could open a PR just with the refactor commits but then this PR would need to depend on that new one. Let me know.