Add extractor field in base language QL packs

[hmakholm]

The functionality of https://git.semmle.com/Semmle/code/pull/36591 depends on extractor fields being present in the base language packs, rather than just in test packs.

They shouldn't do any harm and have been tolerated by the qlpack.yml parser since release 2.0.1 of the CodeQL CLI.

[hmakholm]

Actually we've been holding off on adding extractor fields to the test packs, and instead depended on a semi-undocumented hack that falls back to queries.xml files for getting extractor IDs for test.

However, since I've now verified that the only CLI release that doesn't understand these extractor fields is 2.0.0 (which there's no reason to try to keep supporting), we might as well add them in the test packs too. That makes documentation easier, because it then doesn't have to explain why the examples in the public repo are different from what we recommand for third-party writers' own tests.