[#568] Add GitHub action to verify the front matter metadata

[jmesnil]

This fixes #568

[jmesnil]

@bstansberry @darranl This PR enables us to automatically validate that a proposal has valid categories in its metadata.
If that's not the case, it will add a invalid-categories label to the PR. Once this is valid, the label would be removed.

I can't provide a test case for this from this PR so I've created a temporary branch at https://github.com/wildfly/wildfly-proposals/tree/github_action_frontmatter. When I opened #575 against this temp branch, you can see the check is failing (with a clear error message at https://github.com/wildfly/wildfly-proposals/actions/runs/9362939230/job/25772708885?pr=575) and the invalid-categories label is added to the PR.

This enhancement would make sure that we have proposals with valid categories before they are merged.
The check only verifies proposals that are modified by the PR.

@darranl With this setup in place, we could expand our metadata in the front matter of the proposal to further validate that a proposal is in a good shape. For example, we could move to the front matter info such as:

• stability-level-- validate that the stability level is set, we could use that when we generate the web site to highlight the proposal (without having to have a [EXPERIMENTAL] in the document title)
• reviewer, outside-perspective -- these fields would point to the right people involved in the proposal
• issue -- the issue tracker for the proposal

Having labels for these various fields would also help discoverability and community participations (what are the experimental proposals? Which proposals is missing an outside-perspective person, etc.)

[bstansberry]

Thanks @jmesnil!