[tag tool] Large feature, support tagging all languages/scripts:

- [ ] add a 'sample text' dropdown to select all `sample_text` keys except `masthead_*` and `poster_*`; if no `sample_text` then use `exemplar_chars.base`. Given use of notdef requested in #8131, users will see a lot of tofu, but that's OK to start; but to address it, then
- [ ] add 'writing system' and 'language' dropdowns to select scripts and languages, similar to the [mock Tomas showed here](https://github.com/google/fonts/issues/6690#issuecomment-2289087284), using data from https://github.com/googlefonts/lang, so users can filter the families (so families that don't support the script/language selected are no longer shown)