Add deprecations test script; run on release branches

[shawnbot]

This adds a deprecations test script (and Actions workflow step, which runs only on release-* branches) that ensures:

1. We have removed all selectors from the generated CSS that are listed in the deprecations data for the current version (in which case, we should remove them from the deprecations data); and
2. We have not removed any selectors that aren't listed in the deprecations data (in which case, we should add them back).

Currently this test is running and failing on this branch because it begins with release- (whoops), and will fail #887 until we merge #736 and #662.

Running script/test-deprecations.js --help explains how it works:

script/test-deprecations.js [options]

  --version <version>    The published version of @primer/css from which to
                         fetch CSS selector stats; default: "latest".
  --bundle <bundle>      The CSS bundle to compare; default: "primer".

Fetches the CSS selectors for the published package and checks that:

1. All selectors listed in deprecations.js for the current local version (in
   package.json) have been deleted.
2. All selectors deleted in the current local version have been listed in
   deprecations.js.

If either check fails, the process exits with an error status (1).

TODO

• Merge Reorganize and lint scripts; remove unused dev dependencies #890. (I cherry-picked this branch from there to keep the diff smaller; see the compare view for a clearer diff.)

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