JS: create downgrades pack

[nickrolfe]

No description provided.

[nickrolfe]

N.B. I haven't tested this at all. For now, somebody could test it by following these instructions (search for the paragraph starting "To test the downgrade script...").

[nickrolfe]

This also includes a downgrade script for the changes introduced in #9457.

[cklin]

N.B. I haven't tested this at all. For now, somebody could test it by following these instructions (search for the paragraph starting "To test the downgrade script...").

I tested it by adding codeql_check_db_downgrades('javascript') to the build file and running target/test/check-db-downgrades-javascript. The test passed, so at least the downgrade is well-formed and applies successfully to an empty database.

Once this PR is merged and the semmle-code/ql submodule bumped, I can put up a PR to add the downgrade script check for JS and hook that up to checks-javascript-language-tests-linux.

[cklin]

LGTM in terms of the general structure of the downgrade pack and the downgrade script.