gg: add is_key_down/1 helper method

vlib/gg/keyboard.v

@@ -0,0 +1,8 @@
+module gg
+
+// is_key_down returns whether the given key is currently pressed.
+// You can use this, if you do not want to implement your own key event handling.
+@[inline]
+pub fn (mut ctx Context) is_key_down(k KeyCode) bool {
+	return ctx.pressed_keys[int(k)]
+}