ci(tmux): Add tmux 3.4 to test matrix

[tony]

See also:

• https://github.com/tmux/tmux/blob/3.4/CHANGES
• ci(tmux): Add tmux 3.4 to test matrix tmuxp#909

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f5c1ec9) 89.10% compared to head (f41b99d) 89.10%.

❗ Current head f41b99d differs from pull request most recent head 377c478. Consider uploading reports for the commit 377c478 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #524   +/-   ##
=======================================
  Coverage   89.10%   89.10%           
=======================================
  Files          36       36           
  Lines        3562     3562           
  Branches      501      501           
=======================================
  Hits         3174     3174           
  Misses        267      267           
  Partials      121      121           

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