add schema validation for relgen

[josh-heyer]

What Changed?

This may not be the nicest way of handling errors, but it has the distinct advantages of allowing for in-editor warnings when using VSCode, which is nicer than dealing with error messages anyway.

also:

• show warnings / errors in GitHub when running from Actions

• allow running from npm

Reviewing

Pull locally, then from VSCode terminal run

npm i

then

npm run releasenotes:generate-all

Should have clickable links to non-compliant files with vaguely relevant line and column numbers. Click them, get reasonable-ish errors shown inline.