Add first draft of typescript definitions

[jportela]

I've created a first draft of the typescript definitions file. I've attached a quick gif showing how it helps on developer workflow, even if the developer is not using Typescript (baked in in Visual Studio Code, but can easily be setup for other editors).

I've relied on the http://pm2.keymetrics.io/docs/usage/pm2-api/ documentation to create the definitions, since there's a lot of comments there I could reuse. However, this might not reflect accurately what's in code, so I'll be making a second pass at the code, making sure it accurately translates what's on the documentation. I might also add other functions that are not documented.

Let me know if this is a good direction to take this into, I hope this is helpful.

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	#3186
License	MIT
Doc PR	https://github.com/pm2-hive/pm2-hive.github.io/pulls

[CLAassistant]

All committers have signed the CLA.

[vmarchaud]

Thanks for looking into this, it definitely is the right direction.
I will try to add few myself and correct some if there is any error :)

[jportela]

as a reference, here's the generated typescript definitions by dts-gen:
https://gist.github.com/jportela/7e22bcd12375d3144e30d4445a144d46