Add workflow to check CAIPs

[ligi]

#181 needs to be merged first

[ritave]

Wouldn't you prefer to use something already built instead of rebuilding a new one?

I've written a re-usable markdown linter for SIPs, which operates on Abstract Syntax Tree of markdown and allows writing custom rules per repository.

For example, this rule checks any external links for 400 statues by visiting specific nodes in AST.

I could release the linter into MIT and publish on NPM and allow it to load CAIPs specific plugins.

• Here's a simple rule that verifies the filename matches sip-x.md format.
• Here's a rule that verifies existence of proper copyright. It takes the whole AST and matches the last nodes to template.
• A more complicated one checks the front-matter to verify it abides by the SIP process.

Especially since most of CAIPs operate on TypeScript's types and general Web ecosystem assumptions. And while I like Kotlin since I've been writing in it professionally for few years, it's not the language users of this repo will be most familiar with.

[ligi]

This is reused - I used it for DIPs before. Link checker there I used another one before - gaurav-nelson/github-action-markdown-link-check@v1 - but actually removed it from this step as Link checking is not deterministic.
Feel free to add yours afterwards - this already found stuff and can guard until something else is in place.

[ritave]

I'm good with whichever if we're reusing and not writing one anew again.

[bumblefudge]

#181 merged. Maybe force-push to see if CI will re-run?

[obstropolos]

@ligi - is this all working?

[obstropolos]

@ligi - doing some quick housekeeping - is this all working / some conflicts to resolve.