Repo: Merge codecov report from ast-spec into typescript-estree's report

[JoshuaKGoldberg]

Suggestion

#6240 shows an example of a PR where changes in one package are given unit test coverage by changes in another package. Is that something we want factored into the codecov reports? At first I wanted it, but now I'm wondering if we should consider coverage just from within each package's own unit tests...

If we do want this, from @RebeccaStevens's #6240 (comment):

Codecov can merge multiple reports. I've already set this up in ts-api-utils to merge the reports from different TypeScript versions.

See https://docs.codecov.com/docs/flags for more details on how it works.

[bradzacher]

I think the only one we want to merge would be tests from ast-spec covering the code in typescript-estree - because those are the actual tests for that package!

However I don't think we would want our utility packages to be "covered" by the tests in eslint-plugin - implicit tests via usage seem like they aren't a great marker for test coverage.