n-api: implement async helper methods

Based on the async methods we had in abi-stable-node before the napi
feature landed in node/master. Changed this set of APIs to handle
error cases and removed a lot of the extra methods we had for setting
all the pieces of napi_work opting instead to pass all of those as
arguments to napi_create_async_work as none of those parameters are
optional except for the complete callback, anyway.

Renamed the napi_work struct to napi_async_work and replace the
struct itself with a class which can better encapsulate the object
lifetime and uv_work_t that we're trying to wrap anyway.

Added a napi_async_callback type for the async helper callbacks
instead of taking raw function pointers and make this callback take a
napi_env parameter as well as the void* data it was already taking.

Call the complete handler for the async work item with a napi_status
code translated from the uvlib error code.

The execute callback is required for napi_create_async_work, though
complete callback is still optional.

Also added some async unit tests for addons-napi based on the
addons/async_hello_world test.

PR-URL: https://github.com/nodejs/node/pull/12250


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Hitesh Kanwathirtha <hiteshk@microsoft.com>