Add fallback cli option

[kutu]

Currently const lv_font_t my_font is const, and fallback field can only be assigned by editing my_font.c directly.

after this line https://github.com/lvgl/lv_font_conv/blob/49559663c82cf88939b10b67de028569852b0a1f/lib/writers/lvgl/lv_table_head.js#L77 add

.fallback = &lv_font_montserrat_12,

where lv_font_montserrat_12 will be fallback cli option