Editor suggestions when writing .plugin.js files

[yeegor]

Overview

It is not convenient not having any help when writing .plugin.js files in the project. It'd be useful to at least know which namespaces have which classes and which members.

This may be achievable by applying TS. A register function's mockup exists, but it should be thought through more in order to provide a solution more consistent with the current code of all of the projects.

[yeegor]

In this track, the following functionality would be very useful as well:

1. Ctrl + click on namespace to open locations (at least the original file)
2. On any piece of functionality, show which namespaces are modifying it - right in the file (as git lens extension does)