Add TODO@version stylelint rule (local)

[shawnbot]

This adds a new local stylelint rule that looks for comments in the following form:

// TODO@<version>: <message>

...and compares the <version> with either the current version in package.json or the PRIMER_VERSION environment variable (useful for testing!) and, if the comment version is >= the current version (in semver), fails the test.

Rationale

This offers a more ad-hoc and enforceable way of queueing up tasks for a forthcoming release. I've included #945 in this PR as a test case, which you can validate with:

PRIMER_VERSION=14.0.0 npm run stylelint

...which outputs:

src/navigation/underline-nav.scss
✖ Unresolved TODO comment: "remove &.selected" (expected to be resolved in "14.0.0") (primer-css/TODO)

The idea is that if we merge this into #925, it should fail until we remove the &.selected from the selector and the // TODO@14.0.0: remove &.selected comment.

@simurai what do you think? Is this a good idea, or am I over-thinking things?

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[simurai]

Is this a good idea, or am I over-thinking things?

I think it's great for fixing things in patches/minors and then not forget about cleaning up later.

Although not sure about removing .selected for UnderlineNav just yet, see https://github.com/primer/css/pull/945/files#r334718266.

[shawnbot]

After taking a fresh look at this, I think having a separate script with different error output is unnecessary here, since the lint step in our Actions setup will output the same error. In fact, if it's working right, we'd never get to this standalone test because lint will short-circuit the whole run. So I'm going to delete script/test-todos.js and revert df9c926 before merging this.