Vendor web sys

[andyleiserson]

The only thing that has changed in the parts of web-sys we use is that the github URLs for wasm-bindgen are updated. But this memorializes the bindings as generated from the authoritative copy of the web-sys repo, and not my fork, now that wasm-bindgen/wasm-bindgen#4508 is merged.

Review note: I made some changes to the implementation of the vendor-web-sys xtask, these are in a separate commit, or just scroll to the very bottom of the diff view. Everything else is from vendoring.

Testing
Tested operation of the vendor-web-sys command locally. Other than that, will let CI do its thing.

Squash or Rebase? Rebase (but please squash fixups first, if any)

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.
• If this contains user-facing changes, add a CHANGELOG.md entry.

[cwfitzgerald]

D0pe