feat(keyboard): add user-defined modes (#2651)

As discussed in https://forum.lvgl.io/t/localized-keyboard-layouts/6734,
this adds further `..._USER_N` keyboard modes that can be used to
implemented more keyboard layouts for locales that need more than 4
layers.

src/extra/widgets/keyboard/lv_keyboard.h

@@ -41,6 +41,10 @@ enum {
     LV_KEYBOARD_MODE_TEXT_UPPER,
     LV_KEYBOARD_MODE_SPECIAL,
     LV_KEYBOARD_MODE_NUMBER,
+    LV_KEYBOARD_MODE_USER_1,
+    LV_KEYBOARD_MODE_USER_2,
+    LV_KEYBOARD_MODE_USER_3,
+    LV_KEYBOARD_MODE_USER_4,
 };
 typedef uint8_t lv_keyboard_mode_t;