Allow using new CodeQL languages without updating the CodeQL Action

[henrymercer]

This PR revives #779, adding a few new features:

• Correctly handle language aliases (these didn't exist at the time)
• Determine whether or not a language is traced by looking for its tracing-config.lua file — this is what we do in the CLI too
• Simplify the parsing logic in config-utils.ts

Commit-by-commit review recommended as one commit has a large diff from renaming the Language enum to KnownLanguage.

Caveats:

• We don't attempt to make language handling in the start-proxy Action generic, because this Action can't assume access to the CodeQL CLI, which is the source of truth. However this is probably OK because the default behaviour of this Action, i.e. not to proxy anything, is OK for new languages.

Merge / deployment checklist

• [ ] Confirm this change is backwards compatible with existing workflows.
• [ ] Confirm the readme has been updated if necessary.
• [ ] Confirm the changelog has been updated if necessary.