refactor: Split finish between Global and CommandEncoder

[andyleiserson]

In the encode-on-finish changes, I had put all of the logic related to finishing encoding in Global::command_encoder_finish. Since non-ID-using clients want to bypass Global and call methods on the underlying objects directly, this moves all of the real logic into CommandEncoder::finish and leaves just object retrieval from an ID in Global.

Testing
n/a

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