feat: Support @attrs.frozen and @attrs.mutable

[ngnpope]

The attrs package has two additional decorators that are aliases for @attrs.define which should be included:

• attrs.mutable() is an alias for attrs.define()
• attrs.frozen() is an alias for define(frozen=True)

See https://www.attrs.org/en/stable/names.html#tl-dr

[codecov]

Codecov Report

Merging #133 (d97a8af) into main (8e0d351) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #133   +/-   ##
=====================================
  Coverage   98.7%   98.7%           
=====================================
  Files          3       3           
  Lines        469     472    +3     
=====================================
+ Hits         463     466    +3     
  Misses         6       6           

Impacted Files	Coverage Δ
flake8_type_checking/checker.py	98.5% <100.0%> (+<0.1%)	⬆️
flake8_type_checking/constants.py	100.0% <100.0%> (ø)

[sondrelg]

Lovely, thanks @ngnpope 👏