This is an issue to track: how to support embedded languages. For starters I'm thinking about "<script>" tags in HTML.
I will try to do the research if something like this exists in the wild.
Personally I would start with just diagnostics and later we could improve this support for other methods.
For now I've found this issue from other repo: quarto-dev/quarto-nvim#2
and they have proposed a sulution that we could have hidden buffer with scripts tag content and we attach LSP to that buffer and sync the LSP info between buffers. I think there is something to it.
Also I think it would be great selling point for this plugin because no other tsserver LSP cannot do that right now (especially "typescript-language-server")