fix(naga): Improve validation of non-constructible types

[andyleiserson]

Somewhat a follow-up to #8741. Fixes #4720, and fixes the test cases in #7393, but not the issue itself.

After studying the spec more and the related CTS test cases, I think the necessary zero-value validation can be simplified to "type must be constructible". So this changes the zero-value validation from #8741 to operate that way.

Also adds other validation in the WGSL front-end for non-constructible types in places they are not permitted.

This change is adjacent to the template-list discovery change (#8386), and may have some conflicts, but as best I could tell the two changes don't directly overlap.

Testing
Enables CTS tests and adds naga wgsl_errors and validation tests.

Squash or Rebase? Squash

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• Need to add a CHANGELOG.md entry.