Add rule to upgrade type alias annotations to keyword (UP040) (#6289)

Adds rule to convert type aliases defined with annotations i.e. `x:
TypeAlias = int` to the new PEP-695 syntax e.g. `type x = int`.

Does not support using new generic syntax for type variables, will be
addressed in a follow-up.
Added as part of pyupgrade — ~the code 100 as chosen to avoid collision
with real pyupgrade codes~.

Part of #4617 
Builds on #5062

scripts/check_docs_formatted.py

@@ -82,6 +82,7 @@ KNOWN_PARSE_ERRORS = [
     "missing-newline-at-end-of-file",
     "mixed-spaces-and-tabs",
     "no-indented-block",
+    "non-pep695-type-alias",  # requires Python 3.12
     "tab-after-comma",
     "tab-after-keyword",
     "tab-after-operator",