Project Management: Add validation for missing GitHub labels configuration

[aduth]

What?

• Adds validation that the "auto-labeler" configuration includes entries for every package
• Adds missing configuration if labels exist
• Exempts packages that are unlabeled

Builds on: #52729

Why?

I noticed #76218 was not initially labeled with "[Package] UI". I didn't realize that this was manually managed with configuration in .github/labeler.yml. While I could have easily just added the configuration, it seems that we should want to keep these in sync with what exists in packages/. With this workflow, someone adding a new package would see a CI failure that hints them toward updating the labeler configuration.

How?

Adds a script and related CI job which validates that each package in packages/ is configured in .github/labeler.yml.

I added some exemptions to get things passing with minimal effort, but I might just go ahead and create these labels so we can have as few exceptions as possible.

Testing Instructions

Observe that the initial commit fails CI because it didn't have the updated configuration.

You can also clone it directly into trunk and validate:

git co github-labeler-missing -- .github/scripts/validate-labeler-config.sh
chmod +x .github/scripts/validate-labeler-config.sh
.github/scripts/validate-labeler-config.sh

[...]
The following packages are missing from .github/labeler.yml:
  - packages/abilities
  - packages/admin-ui
  - packages/annotations
  - packages/asset-loader
  - packages/babel-plugin-import-jsx-pragma
  - packages/babel-plugin-makepot
  - packages/babel-preset-default

[github-actions]

Flaky tests detected in 9fab855.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/22779239332
📝 Reported issues:

• [Flaky Test] Custom Completer: should insert option between two other words #48566 in /test/e2e/specs/editor/various/autocomplete-and-mentions.spec.js
• [Flaky Test] should move focus back to the Publish panel toggle button when canceling #76179 in /test/e2e/specs/editor/various/publish-panel.spec.js

[aduth]

Fails as expected prior to fixes: (see logs)

[github-actions]

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Co-authored-by: aduth <aduth@git.wordpress.org>

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