Add support for methods with func type comment excluding self/cls

[stroxler]

Summary

PEP 484 doesn't really specify carefully how function type
comments should work on methods, but since usually the type of
self / cls is automatic, most use cases choose to only annotate
the other arguments.

As a result, this commit modifies our codemod so that non-static
methods can specify either all the arguments, or all but one of
them. We'll correctly zip together the inline and func-type-comment
types either way, typically getting no type for cls or self.

We accomplish this by using matchers to trigger the visit
method for FunctionDef rather than using visit_FunctionDef, which gives
us enough context to determine when a function def is a regular
function versus a method (plus also matching the decorators against
@staticmethod, so that we trigger the normal function logic in
that case).

Test Plan

I added a new test verifying various method cases: static and non-static,
with and without annotations of self / cls in the func type comment,
with and without inline type comments.

[codecov-commenter]

Codecov Report

Merging #622 (8cf00a7) into main (d35b6a5) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #622      +/-   ##
==========================================
- Coverage   94.78%   94.78%   -0.01%     
==========================================
  Files         245      245              
  Lines       25256    25281      +25     
==========================================
+ Hits        23940    23963      +23     
- Misses       1316     1318       +2     

Impacted Files	Coverage Δ
libcst/codemod/commands/convert_type_comments.py	96.20% <100.00%> (+0.18%)	⬆️
...demod/commands/tests/test_convert_type_comments.py	97.10% <100.00%> (+0.17%)	⬆️
libcst/_nodes/statement.py	95.05% <0.00%> (-0.10%)	⬇️
libcst/_nodes/tests/test_atom.py	97.56% <0.00%> (ø)
libcst/_nodes/tests/test_match.py	100.00% <0.00%> (ø)
libcst/_nodes/tests/test_tuple.py	94.73% <0.00%> (ø)

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