Both in progress.

Language server:

https://github.com/nim-lang/langserver

https://github.com/nim-lang/langserver/issues/1

As for a REPL, there is already `nim secret`, but there is work being done to create a better interactive environment.

https://github.com/nim-lang/Nim/issues/8927

I use Org Babel in Emacs as a "REPL", and an Org Babel package for Nim exists: ob-nim[0], and it's awesome.

For folks uninitiated about Org Babel, it allows one to write small code snippets and quickly evaluate the results and have them inserted below. I keep a nim-scratch.org handy to quickly try out new snippets. Unlike traditional REPL's, this approach allows me to keep on saving new snippets and their results too!

Here's how it looks: https://i.imgtc.com/FAnay93.png

Here's another example: https://i.imgtc.com/xLfiR6p.png. The beauty of this approach is that then I can simply export that my blog[1] (you can correlate 1-1 how that screenshot matches with the content on that blog) :).

[0]: https://github.com/Lompik/ob-nim

[1]: https://scripter.co/notes/nim/#nim-help

Language server is in construction, but you can use gdb with VSCode quite well within the editor.