LSP: Proof decomposition code actions.

[kape1395]

To try this out, build and install (make all install) TLAPM from this branch. The existing VSCode extension works without changes.

• The majority of the decomposition suggestions are proposed for a QED step.
• Tests are still missing, working on that.
• I expect to have misc adjustments, e.g., when to suppress the decompositions, which facts to include/drop from a proof, etc.

Please report your impressions on what could be improved.

This is not yet ready for a code review. I will clean it up before making the PR non-draft.