feat: Adds support for PEP-649 via type-checking-py314plus setting.

[Daverball]

Closes #206

This adds a new setting type-checking-py314plus for enabling PEP-649 semantics when analyzing annotations.

Currently this setting has the same effect as adding a from __future__ import annotations import to every file we're analyzing, although in the future the detailed semantics may diverge ever so slightly, if necessary.

We may also consider adding a rule for removing from __future__ import annotations imports when Python 3.14+ mode is active, although that is a little more controversial, since the runtime behavior is still different in 3.14+ when that import is present, so removing it can cause subtle breakages in code that was previously working, albeit only rarely. That rule may also overlap with other plugins, since there are already other plugins for removing outdated futures imports.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.4%. Comparing base (7d24f79) to head (06c3cff).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #207   +/-   ##
=====================================
  Coverage   96.4%   96.4%           
=====================================
  Files          4       4           
  Lines        959     968    +9     
=====================================
+ Hits         925     934    +9     
  Misses        34      34           

Files with missing lines	Coverage Δ
flake8_type_checking/checker.py	96.1% <100.0%> (+<0.1%)	⬆️
flake8_type_checking/plugin.py	100.0% <ø> (ø)

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