[red-knot] support invalid syntax without panics

[carljm]

The ruff parser is error-resilient and will generate a best-effort AST for any input, but red-knot currently assumes a valid AST in some places, and this can cause panics if it's run over an invalid AST.

We should do best-effort type-checking for invalid ASTs, and never panic.

The trick is to do this while, as much as feasible, still maintaining some useful internal invariants, and failing fast (panic is good in this case!) if those invariants are violated, rather than allowing programming errors to pass silently and result in type-checking bugs.

[carljm]

#13701 is a useful reference on what changes were needed as of a few days ago to avoid panics on the Python files present in the ruff repo. At least some of those are due to invalid syntax.