Emit a message for unhashable dict keys

[mbyrnepr2]

Current problem

The following example leads to a TypeError because the key is unhashable.
pylint currently emits nothing for this:

{[1, 2, 3]: "tomato"}

Desired solution

Emit a message indicating the TypeError.

Additional context

Noticed this while writing documentation for the unhashable-dict-key checker: #7033 (comment).

Note the existing unhashable-dict-key checker emits a message when the key being looked up is unhashable; not the keys in the dict itself. I.e.:

{"tomato": "red"}[["a", "b", "c"]]  # [unhashable-dict-key]