feat(float): allow enabling mouse for non-focusable window (#30844)

Problem:  Cannot allow mouse interaction for non-focusable float window.
Solution: Add a "mouse" field to float window config.

src/nvim/grid_defs.h

@@ -80,8 +80,8 @@ struct ScreenGrid {
   // whether the compositor should blend the grid with the background grid
   bool blending;
 
-  // whether the grid can be focused with mouse clicks.
-  bool focusable;
+  // whether the grid interacts with mouse events.
+  bool mouse_enabled;
 
   // z-index: the order in the stack of grids.
   int zindex;