macOS: use KeyboardShortcut rather than homegrown KeyEquivalent

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.

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/Helpers/EventModifiers+Extension.swift

@@ -0,0 +1,27 @@
+import SwiftUI
+
+// MARK: EventModifiers to NSEvent and Back
+
+extension EventModifiers {
+    init(nsFlags: NSEvent.ModifierFlags) {
+        var result: SwiftUI.EventModifiers = []
+        if nsFlags.contains(.shift)     { result.insert(.shift) }
+        if nsFlags.contains(.control)   { result.insert(.control) }
+        if nsFlags.contains(.option)    { result.insert(.option) }
+        if nsFlags.contains(.command)   { result.insert(.command) }
+        if nsFlags.contains(.capsLock)  { result.insert(.capsLock) }
+        self = result
+    }
+}
+
+extension NSEvent.ModifierFlags {
+    init(swiftUIFlags: SwiftUI.EventModifiers) {
+        var result: NSEvent.ModifierFlags = []
+        if swiftUIFlags.contains(.shift)     { result.insert(.shift) }
+        if swiftUIFlags.contains(.control)   { result.insert(.control) }
+        if swiftUIFlags.contains(.option)    { result.insert(.option) }
+        if swiftUIFlags.contains(.command)   { result.insert(.command) }
+        if swiftUIFlags.contains(.capsLock)  { result.insert(.capsLock) }
+        self = result
+    }
+}