zed does not yet support external lsp configuration officially.
According to the source code released yesterday, it seems implemented in the form of a wasm plugin.
- https://github.com/zed-industries/zed/blob/1c2859d72b7fe02e7c77cc54f3eadab114e3e92a/crates/zed/src/languages.rs#L232-L245
- https://github.com/zed-industries/zed/blob/1c2859d72b7fe02e7c77cc54f3eadab114e3e92a/crates/zed/src/languages/language_plugin.rs
Since rescript lsp does not have a wasm build, it may require porting implementation into another lang or using an additional toolchain such as javy.
FYI I experimented with getting a wasm build of the rescript compiler using wasm_of_ocaml, and it pretty much works, but has some problems integrating with the external fs, still investigating.