Linter action with LibreCores CI

[wallento]

This adds a beta Github action that is run on pushes and pull requests. It automatically annotates the sources with linter issues.

Nothing extra is needed beyond merging.

As a demo you can see how new PRs will fail: https://github.com/wallento/ibex/pull/4 They will then show the annotation: https://github.com/wallento/ibex/pull/4/commits/bbf8e108bb313655544ebb490ff9816ffb981b87

Removing the current waiver looks like this as a test: https://github.com/wallento/ibex/pull/5/commits/966649b759d146e088b20112503524adc8c8c6fc So, you can see that it will also annotate files outside the changeset.

There will be robustness extensions, but I think it is usable and I would love ibex as beta testers. It can be deactivated or removed as easily as it was added :)

[wallento]

Links are broken by github autolinking, the second one is accessible via https://github.com/wallento/ibex/pull/5