docs: WebAssembly build instructions #40541

Adds build instructions for the experimental WASM (`wasm32-emscripten`) target
under the Cross-compiling section of `BUILD.md`.

BUILD.md

@@ -593,3 +593,23 @@ Also relevant for webassembly (WASM) build.
   cross-compiling or "-Dhost=" (empty string) to assume that target binaries
   can run on the host during the build process (e.g. if target is x86 on a
   x86_64 system, or if emulation set up with binfmt or similar)
+
+### WebAssembly (experimental)
+
+Neovim can be cross-compiled to WebAssembly using the `wasm32-emscripten` target.
+
+- Requires the [Emscripten SDK](https://emscripten.org/docs/getting_started/downloads.html) (`emsdk`).
+- After installing and activating it, pass its sysroot to `-Demscripten-sysroot`, e.g. `$EMSDK/upstream/emscripten/cache/sysroot`.
+
+To build the WebAssembly target:
+
+```bash
+zig build nvim_bin \
+  -Dtarget=wasm32-emscripten \
+  -Demscripten-sysroot=$EMSDK/upstream/emscripten/cache/sysroot
+```
+
+This currently produces:
+
+- `nvim.wasm`, the WebAssembly module.
+- `nvim.js`, the JavaScript loader generated by Emscripten.