[nix] add emscripten dependency

Summary: Add WebAssembly compiler (source)

Test Plan:

1. Exit from nix
2. nix develop - should install package
3. Check if tool is available emcc -v

emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.17-git
clang version 14.0.6
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /nix/store/m9xq3vnz2g165hypy010rbwab8iskcv5-emscripten-llvm-3.1.17/bin

Reviewers: jon, ashoat, O1 Nix, bartek

Reviewed By: jon, ashoat, O1 Nix, bartek

Subscribers: bartek, tomek

Differential Revision: https://phab.comm.dev/D8375