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