I’m using lsp-lua along with defold-annotations. Additionally, I use dmSDK with LSP-clangd. This setup is more than enough for me.
Defold extensions are relatively small, and their API is easy to remember. I don’t use anything additional for them.
What kind of extensions do you mean?
If you are talking about Lua-based extensions, as far as I know, the only way to do it by downloading and putting them in the project folder manually. Then, lsp-lua can handle the rest.
For C/C++ extensions, I don’t know a solution.