Large rework the checks action

[mithro]

This is a large rework of the SymbiFlow/actions/checks action.

It does the following;

• Rewrites the action as a Python script and adds some Python tests fixing #21.
• Adds tests for the various exclude functionality.
• Reworks the input configuration, fixing #17.
• Creates annotations for errors when they are discovered.

[mithro]

While the annotation output looks a bit funky in the action output, it seems to correctly appear in the pull request output;