API Documentation

As far as I can tell there is no easily accessible API documentation outside of the source code.

However, the typescript declaration file has most functions documented in addition to its types.

I suggest using `typedoc` to generate documentation. Users can do this themselves or this can be hosted online for free. An example: https://funkia.github.io/hareactive/

This command worked for me:

```
typedoc —out godot-ts-doc ./godot.d.ts —ignoreCompilerErrors —module commonjs —target es6 —includeDeclarations ./godot.d.ts
```

Additionally, I am available to help with documentation beyond this. This is a high-quality project and I would like to help it be as visible and accessible as possible.