feat: add KeyCombo.Ctrl and KeybindingRule.to_keybinding

[tlambert03]

closes #229

This PR adds two more KeyMods: KeyMod.Ctrl and KeyMod.Meta

• CtrlCmd: command on a mac, control on windows
• Shift: shift key
• Alt: alt/option key
• WinCtrl: meta key on windows, control key on mac
• Ctrl: control key, regardless of OS (NEW)
• Meta: command key on a mac, meta key on windows (NEW)

most of the time, you will want to use CtrlCmd and WinCtrl, since they do what is usually "expected" when the keycode is converted to a Keybinding object. But in cases where you just always want the control key, or meta key, regardless of platform, the new codes can be used. Note: it can be confusing if you combine Ctrl with CtrlCmd ... (it would be just ctrl on windows, but cmd+ctrl on macos) so it's definitely not recommended.

It also adds some conveniences for debugging. It adds a public KeyBindingRule.to_keybinding method that takes in an optional os. so one can easily test how these things are resolved, on any platform

from app_model.types import KeyBindingRule, KeyCode, KeyMod, OperatingSystem

rule = KeyBindingRule(primary=KeyMod.CtrlCmd | KeyCode.KeyX)
os = OperatingSystem.WINDOWS
print(rule.to_keybinding(os=os).to_text(os=os))

[tlambert03]

@dalthviz, would appreciate your review. (also, added you to collaborators here)

I'm not sure I'm convinced on the need for these two additional KeyMods... but I do think that the other stuff in the PR is generally useful.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (a0d95b1) to head (51249e5).

Additional details and impacted files

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##              main      #230   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           31        31           
  Lines         1916      1917    +1     
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+ Hits          1916      1917    +1     

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[dalthviz]

Thanks @tlambert03 for the invitation! I think the only suggestion from my side would be to somehow see a way to add the info you put over the PR description in the documentation. Maybe adding some of that info over the KeyMod class docstring could be a way of achieving that? Or maybe is there a way for the docs to show a link to go to the source so people can check the inline comments on the enum values to better graps who the enum values interact with the different platforms/operative systems? 🤔