[wgsl-in] Split into multiple files

[SparkyPotato]

In #2075, we left all the changes in one file to make reviewing easier. However, this made front/wgsl/mod.rs huge (5,447 lines), and quite challenging to follow.

This PR splits each pass into its own module, bringing both parse/mod.rs and lower/mod.rs to a more manageable ~2,000 lines. Other than uses, everything has been copy-pasted.

Since the frontends now do more than parse, I also renamed them to Frontend instead of Parser to make the intent more clear: #2075 (comment)

[jimblandy]

This looks great, but let's fix those doc links.

[SparkyPotato]

Oops, missed that.

Rustdoc seems to be acting weird, however.
Linking crate::front::wgsl::lower::ExpressionContext::local_table directly errors, but if I use crate::front::wgsl::lower::ExpressionContext, it stops complaining.

[jimblandy]

This looks good, just some minor points.

Acknowledging Teo's suggestion in #2075 that all the front ends should use Frontend instead of Parser, I have to say I would also approve a PR that just left GLSL alone.

[jimblandy]

Looks good to me. Just minor comments.

I don't know how @JCapucho is going to feel about the GLSL changes. If he's not happy with them, we can back them out.

[jimblandy]

Looks good - thanks!

[jimblandy]

Clippy got clevererer.

[jimblandy]

Filed #2229 for the Clippy problems.

[jimblandy]

rebased on merged #2229