N-API: An api for embedding Node in applications

**Is your feature request related to a problem? Please describe.** Right now there isn't a documented/stable way to use Node as a shared library inside of an application. Were one to be made using N-API, this would open up using Chakra in addition to V8 in an application. **Describe the solution you'd like** I would like for there to be stable APIs in `node_api.h` for creating/managing a Node environment. A function that does this could hypothetically look like: ```cpp NAPI_EXTERN napi_status napi_create_env(int* argc, const char** argv, napi_env* env); // Start the node event loop NAPI_EXTERN napi_status napi_run_env(napi_env env); // Cleanup (e.g. FreeIsolateData, FreeEnvironment and whatever else needs to be ran on teardown) NAPI_EXTERN napi_status napi_free_env(napi_env env); ``` The embedder could get this environment's libuv loop using `napi_get_uv_event_loop`. But I would also like to have open the possibility of providing my own libuv loop that I have control over to help integrate with other event loops (e.g. Qt's event loop). This could look like: ```cpp NAPI_EXTERN napi_status napi_create_env_from_loop(int* argc, const char** argv, napi_env* env, struct uv_loop_s* loop); ``` Keeping the event loop going (using `uv_run` on the env's loop) would then be the embedder's responsibility. Also, right now methods like `node::CreateEnvironment` seem to always jump into a REPL, unless you provide a string to evaluate or a file to run. Tweaks to help make this nicer to use for embedding will have to be made. These APIs are just hypothetical, and will probably change when an actual attempt to implement them is made. I am up to trying to implement this, but I would like to see what kind of discussion happens first and what other ideas people have before I start. **Implementation Progress** - [ ] Create a clean non-NAPI way to use Node embedded - [ ] Create NAPI functions for creating and managing environments - [x] Create a NAPI function for evaluating a string (…