macos: rectangle select only requires option + drag

Fixes #2537

This matches Terminal.app. iTerm2 requires cmd+option (our old
behavior). Kitty doesn't seem to support rectangle select or I couldn't
figure out how to make it work. WezTerm matches Terminal.app too.
Outside of terminal emulators, this is also the rectangular select
binding for neovim.

src/surface_mouse.zig

@@ -113,14 +113,19 @@ fn eligibleMouseShapeKeyEvent(physical_key: input.Key) bool {
         physical_key.leftOrRightAlt();
 }
 
-fn isRectangleSelectState(mods: input.Mods) bool {
-    return mods.ctrlOrSuper() and mods.alt;
-}
-
 fn isMouseModeOverrideState(mods: input.Mods) bool {
     return mods.shift;
 }
 
+/// Returns true if our modifiers put us in a state where dragging
+/// should cause a rectangle select.
+pub fn isRectangleSelectState(mods: input.Mods) bool {
+    return if (comptime builtin.target.isDarwin())
+        mods.alt
+    else
+        mods.ctrlOrSuper() and mods.alt;
+}
+
 test "keyToMouseShape" {
     const testing = std.testing;