[nix] add emscripten dependency

Summary: Add WebAssembly compiler ([source](https://emscripten.org/index.html#))

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

nix/dev-shell.nix

@@ -16,6 +16,7 @@
 , corrosion
 , darwin
 , double-conversion
+, emscripten
 , folly
 , fmt
 , glog
@@ -55,6 +56,7 @@ mkShell {
     awscli2
     shellcheck
     terraform
+    emscripten
 
     # android
     openjdk11