890ba725d9
This moves all INVALID_ASSIGNMENT lints related to unpacking over to the new diagnostic model. While we're here, we improve the diagnostic a bit by adding a secondary annotation covering where the value is. We also split apart the original singular message into one message for the diagnostic and the "expected versus got" into annotation messages.
Markdown files within the mdtest/ subdirectory are tests of type inference and type checking;
executed by the tests/mdtest.rs integration test.
See crates/red_knot_test/README.md for documentation of this test format.