Indeed, Textadept was one of my go-to editors for this, and scripting it in Lua was fantastic! But again, it's not quite what I am after, and I'm obviously explaining it badly :)

Basically, I'd like the editor to understand the AST of the language(s, including mixed-language files, which is really hard) and be able to hook into parts of that AST to say "while this is a string, it's really this weird DSL that hooks into Javascript, so look for symbols here, here and here".

Its definitely possible to make Vim, Emacs and TA work this way, but its a lot more work than I had in mind, when I tried to extend them (well, not Emacs. It might be best at this, I'm just not a good emacs user) to achieve what I was after at the time, which was understanding the dependency injection annotation patterns from Symfony in PHP.

I think one could get pretty close with LSPs and the powerful AST parsers we have nowadays. Though doing it fast might be difficult!