Support extending revset functionality with custom functions and programmatic filters

[torquestomp]

This doesn't support everything an extension might want to do with the revset language, but it covers a large portion of it and for much less internal cost than the previous proposal.

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, demos/)
• I have updated the config schema (cli/src/config-schema.json)
• I have added tests to cover my changes

[jonathantanmy]

Also is there a way to test the function and filter extensibility? I see that there is a CLI example that shows how to create a new filter, but that doesn't seem to cover the function ability, and does not seem to be exercised in a test.

[torquestomp]

Also is there a way to test the function and filter extensibility? I see that there is a CLI example that shows how to create a new filter, but that doesn't seem to cover the function ability, and does not seem to be exercised in a test.

Unfortunately I don't think there's currently a good way to set up tests for extension behavior; the low level revset_engine tests do not have enough context to produce a Store, and the high-level tests run jj directly which does not have extensions loaded in. Maybe we need some way to run example binaries instead of the main binary in tests

[yuja]

Looks generally good to me, thanks!