feat: add implementation rule

[barmac]

Closes #4

[barmac]

On the other hand, the implementation details are not rendered on a diagram, so it doesn't seem to make sense to use pictures to visualise the rule. Still, there is no option in the tool to opt-out from generating images for selected diagrams.

The documentation does not include an image due to the fact that difference is not rendered on the diagram. This is also the reason why I resigned from the diagram examples and used code snippets instead.

[nikku]

Two small comments.

[barmac]

Got feedback from @ThorbenLindhauer that we should disallow missing implementation for the Message Throw/End Events. So this would be a way to promote best practices.

From readme:

Camunda modeling guidelines, packed as a bpmnlint plug-in.

In that case, I think it makes sense to be more strict than the engine.

[nikku]

Let's implement the strict message event definition validation (#11 (comment)) and we're good I think!

[nikku]

I think you removed most of the rough edges.

One thing I thought about: How much sense does it make to activate this rule in non-executable processes?

In BPMN we distinguish between these. Non executable processes are meant for documentation only and not picked up by the engine. I'd assume I can do anything in there, with no such limit.

[barmac]

Yeah, that makes sense. I haven't thought about it. I'll extend the test case to not bug the user about non-executable processes.

[barmac]

Now we ignore non-executable processes.

[nikku]

Very nice 🍰 🎉