Prevent final reassignment in match case (#19496)

Fixes #19507

This PR adds a check to prevent reassignment of final variables in a
match statement.

Currently, the following passes without an error:
```Python
from typing import Final
FOO: Final = 8

a = 10

match a:
    case FOO:
        pass
print(FOO)  # FOO is reassigned, prints 10
```

MyPy already checks that the type of FOO isn't changed if used like
this. I added a check in the same place that makes sure it's not `Final`
either.

Since this tests the match syntax, I put the test where I did. If it's
not the appropriate place for it I will move it as instructed :)

Author: omerhadari

mypy/checker.py

@@ -5577,6 +5577,8 @@ def infer_variable_types_from_type_maps(
                 previous_type, _, _ = self.check_lvalue(expr)
                 if previous_type is not None:
                     already_exists = True
+                    if isinstance(expr.node, Var) and expr.node.is_final:
+                        self.msg.cant_assign_to_final(expr.name, False, expr)
                     if self.check_subtype(
                         typ,
                         previous_type,

test-data/unit/check-python310.test

@@ -2839,3 +2839,13 @@ match value_type:
     case _:
         assert_never(value_type)
 [builtins fixtures/tuple.pyi]
+
+[case testAssignmentToFinalInMatchCaseNotAllowed]
+from typing import Final
+
+FOO: Final[int] = 10
+
+val: int = 8
+match val:
+    case FOO:  # E: Cannot assign to final name "FOO"
+        pass