Support environment variables

[devappd]

Useful for EMCC_LOCAL_PORTS

• Specify environment variable in emscripten.settings.js.
  • Top-level to project (i.e., sibling to type: "cmake" etc.), or specify per step?
• Pass environment variable to emsdk.run()
  • Copy process.env, append our variable, then specify child_process.spawn({ env: ourEnv })