red_knot_python_semantic: migrate INVALID_ASSIGNMENT for shadowing

We mostly keep things the same here, but the message has been moved from
the annotation to the diagnostic's top-line message. I think this is
perhaps a little worse, but some bigger improvements could be made here.
Indeed, we could perhaps even add a "fix" here.

crates/red_knot_python_semantic/resources/mdtest/diagnostics/shadowing.md

@@ -0,0 +1,19 @@
+# Shadowing
+
+<!-- snapshot-diagnostics -->
+
+## Implicit class shadowing
+
+```py
+class C: ...
+
+C = 1  # error: [invalid-assignment]
+```
+
+## Implicit function shadowing
+
+```py
+def f(): ...
+
+f = 1  # error: [invalid-assignment]
+```