Proposal (PlusCal): `action` keyword to write any TLA+ expressions WITHOUT generating an UNCHANGED statement.

[lemmy]

See PR at tlaplus/tlaplus#455

Personal notes (everybody please ignore):
lemmy/PageQueue@4ab1ebb
lemmy/PageQueue@07d2ea5
tlaplus/tlaplus#536

[ahelwer]

Can you provide an example of the updated syntax and how it would be translated, the use case, etc.?

This PR could be built upon to add tests for the new syntax: tlaplus/tlaplus#937