add(input): mouse support

src/event.zig

@@ -3,6 +3,7 @@
 const std = @import("std");
 const terminal = @import("terminal.zig");
 
+const Mouse = @import("mouse.zig").Mouse;
 const Size = @import("size.zig").Size;
 const Key = @import("key.zig").Key;
 
@@ -23,6 +24,8 @@ pub const SystemEvent = union(enum) {
     resize: Size,
     /// Input key event received from the user
     key: Key,
+    /// Mouse input event
+    mouse: Mouse,
     /// Focus event for mouse interaction
     /// TODO: this should instead be a union with a `Size` to derive which container / element the focus meant for
     focus: bool,