I wrote a lisp, a while back, and then later added an LSP for it.

Since lisp is so simple in terms of syntax what I really did was tab-completion, and info-on-hover, for the built-in functions like "car", "cdr", and the primitives I added as part of a more complex standard-library.

In my case I was writing in go and I found an LSP-server package which was trivial to use. So getting the integration with emacs, vim, etc, was really trivial:

https://github.com/tliron/glsp

Unless you want half your C logic to be error propagation, it’ll have to be <setjmp.h>, all its flaws notwithstanding. Manual unwinding can be done tastefully (see Tcl), but in the single-digit-KLOC weight class it’s not really a choice.

while we're marvelling about how small and compact it is, then just replace them (with something uglier and less performant), it's not rocket science