Add lua functions to check mouse selection and copy it somewhere

[bew]

Is your feature request related to a problem? Please describe.
For me, when on Linux (X11), mouse selection copy to clipboard in wezterm is always done with a keybind: Ctrl-Shift-c

But it is always hard to reach, and using it often (because firefox doesn't understand primary selection) is cumbersome, and I don't want to permanently use my precious Alt-c key (very easy to reach!) that I use for copy in all my terminal programs.

Describe the solution you'd like

I want to be able to bind Alt-c to copy mouse selection to clipboard, ONLY when there is an active mouse selection, otherwise send Alt-c to the terminal program.

Needed functions:

• has_mouse_selection() (returns bool)
• get_mouse_selection() (returns string or nil)
• copy_to(destination)

Describe alternatives you've considered
Waiting... but that doesn't work ^^

Or keep using Ctrl-Shift-c keybind, but it's always hard to reach...

[bew]

I missed / forgot about #575, which added a function to get current selection!
combined with the SendKey action, here is the working keybinding:

  {mods="ALT", key="c", action=wezterm.action_callback(function(win, pane)
    local has_selection = win:get_selection_text_for_pane(pane) ~= ""
    if has_selection then
      win:perform_action({CopyTo="ClipboardAndPrimarySelection"}, pane)
    else
      win:perform_action({SendKey={mods="ALT", key="c"}}, pane)
    end
  end)},

[github-actions]

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