Add [coverage] config overrides for the pyrefly coverage globs

[jorenham]

Summary

pyrefly coverage {check, report} used the same project-{includes,excludes} as pyrefly check, so measuring coverage on a different file set (e.g. skipping tests, vendored code, or publically-namespaced private API) required repeatedly specifiying --project-excludes for every pyrefly coverage call.

This adds an optional [coverage] config table with includes and excludes that override the project globs when using pyrefly coverage (and only then).
Each field defaults to its top-level project- counterpart when unset (like it does now).
The --project-excludes flag takes precedence over coverage.excludes, and positional FILES... args also do (just like with the project- globs).
Unknown keys inside [coverage] are reported in a warning message in the same way as with the other config tables.

Test Plan

Config tests added for parsing, path rewriting, and glob resolution (for both the "default" and "coverage" config scopes), and verified that a [coverage] config indeed narrows pyrefly coverage report without affecting pyrefly check and explicit FILES... ("file mode" I believe it's called).

[jorenham]

cc @MarcoGorelli

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅