Validate `bpmnlint:recommended` rules

[nikku]

This PR showcases validation of bpmnlint:recommended rules.

There is clearly a couple of things that we'd need to account for beyond this PR to make this possible. However the PR (screen capture attached) already showcases a few things:

• We shall include the recommended modeling guidelines (bpmnlint:recommended rules) as warnings, not errors. From the automation perspective, these are rather un-critical, good to have things.
• We shall re-work our iconography for warnings (the PR does a poor attempt to fix the existing (unused?) warning icon.
• We shall aim for warnings not being prominently displayed on the diagram (while errors shall be prominently featured. However, warnings are not at all featured at the moment
• We can easily roll this out via a feature toggle. This PR hides the feature behind --enable-bpmnlint-recommended.

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[nikku]

CC @andreasgeier @philippfromme

[nikku]

A related and powerful easy pick could be to allow custom linter rules to be injected via a plug-in.

[andreasgeier]

I agree that best practice rules should throw a warning, not an error. Error = blocks execution Warning = everything else

When adding warnings, we need to do another design round for it. But it shouldn't be a big challenge.

Regarding linting overlays on the diagram: We should let the user choose whether none, one, or both types are preferred to see as overlay. Plus, a basic configuration to show/hide linting results at all (as mentioned in this comment).