Adds a new tool command (and runs it in CI) to check that each package:
- is listed correctly in the repo-level README.md table
- has a CODEOWNERS entry
In the future we could add other things (e.g., auto-label), but these were the main things we've had issues with recently.
Updates README.md and CODEOWNERS to fix failures it found:
- Adds a couple of missing CODEOWNERS
- Expands the web implementation CODEOWNERS to individual packages so that we don't have to special-case handling in the tool
- Fixes some minor mistakes in README.md
- URL-encodes all `:`s in the README.md links (which is why ever line shows as changed); it worked without that in practice, but it should really be encoded, and having it consistently encoded made things easier for the tooling.
