feat: add addKey method to QIconifyIcon

[tlambert03]

This PR adds an addKey method, similar to QIcon.addFile, to QIconifyIcon. It can be used to add icons/colors for various states/modes:

    def addKey(
        self,
        *key: str,
        color: str | None = None,
        flip: Flip | None = None,
        rotate: Rotation | None = None,
        dir: str | None = None,
        size: QSize | None = None,
        mode: QIcon.Mode = QIcon.Mode.Normal,
        state: QIcon.State = QIcon.State.Off,
    ) -> None:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (61e7409) 87.42% compared to head (9aa2a6a) 87.45%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #218      +/-   ##
==========================================
+ Coverage   87.42%   87.45%   +0.02%     
==========================================
  Files          46       46              
  Lines        3356     3355       -1     
==========================================
  Hits         2934     2934              
+ Misses        422      421       -1     

Files	Coverage Δ
src/superqt/iconify/__init__.py	100.00% <100.00%> (+7.14%)	⬆️

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