surface: show .text (i-beam) while shift is held without mouse tracking

I think this is the expected behavoir when a custom OSC22 pointer is set. Once
shift is released it will return to mouse_shape (whatever the pointer
was before shit held).

src/surface_mouse.zig

@@ -96,6 +96,11 @@ pub fn keyToMouseShape(self: SurfaceMouse) ?MouseShape {
             if (isRectangleSelectState(self.mods)) {
                 // Crosshair (rectangle select)
                 return .crosshair;
+            } else if (isMouseModeOverrideState(self.mods)) {
+                // Shift shows an I-beam so selection is obvious even when
+                // the application cursor is not text (OSC 22). Release
+                // restores mouse_shape below.
+                return .text;
             } else {
                 return self.mouse_shape;
             }