macOS: use KeyboardShortcut rather than homegrown KeyEquivalent (#7139)

This replaces the use of our custom `Ghostty.KeyEquivalent` with the
SwiftUI `KeyboardShortcut` type. This is a more standard way to
represent keyboard shortcuts and lets us more tightly integrate with
SwiftUI/AppKit when necessary over our custom type.

This PR should have no user impact. This is just some cleanup for future
work.

Note that not all Ghostty triggers can be represented as
KeyboardShortcut values because macOS itself does not support binding
keys such as function keys (e.g. F1-F12) to KeyboardShortcuts.

This isn't an issue since all input also passes through a lower level
libghostty API which can handle all key events, we just can't show these
keyboard shortcuts on things like the menu bar. This was already true
before this commit.

macos/Sources/App/macOS/AppDelegate.swift

@@ -419,15 +419,15 @@ class AppDelegate: NSObject,
     /// action string used for the Ghostty configuration.
     private func syncMenuShortcut(_ config: Ghostty.Config, action: String, menuItem: NSMenuItem?) {
         guard let menu = menuItem else { return }
-        guard let equiv = config.keyEquivalent(for: action) else {
+        guard let shortcut = config.keyboardShortcut(for: action) else {
             // No shortcut, clear the menu item
             menu.keyEquivalent = ""
             menu.keyEquivalentModifierMask = []
             return
         }
 
-        menu.keyEquivalent = equiv.key
-        menu.keyEquivalentModifierMask = equiv.modifiers
+        menu.keyEquivalent = shortcut.key.character.description
+        menu.keyEquivalentModifierMask = .init(swiftUIFlags: shortcut.modifiers)
     }
 
     private func focusedSurface() -> ghostty_surface_t? {