[plugin] Implement registerDeclarationProvider

[AlexTugarev]

What it does

Implements missing DeclarationProvider API to allow recent versions of Python LS to work nicely.

Fixes #5213

How to test

• install vscode extension for python
• add "python.jediEnabled": false to your preferences
• open any python file and watch the LS being loaded and initialized
• inspect language features at least with import os (check content assist and go to declaration of os)

output channel should contain something like:

Starting Microsoft Python language server.
[Info  - 11:34:23 AM] Analysis cache path: /home/gitpod/.cache/Microsoft/Python Language Server
[Info  - 11:34:23 AM] GetCurrentSearchPaths /home/gitpod/.pyenv/versions/3.7.4/bin/python3 
[Info  - 11:34:23 AM] Interpreter search paths:
[Info  - 11:34:23 AM]     /home/gitpod/.pyenv/versions/3.7.4/lib/python3.7
[Info  - 11:34:23 AM]     /home/gitpod/.pyenv/versions/3.7.4/lib/python3.7/lib-dynload
[Info  - 11:34:23 AM]     /home/gitpod/.pyenv/versions/3.7.4/lib/python3.7/site-packages
[Info  - 11:34:23 AM] User search paths:
[Info  - 11:34:27 AM] Microsoft Python Language Server version 0.3.66.0
[Info  - 11:34:27 AM] Initializing for /home/gitpod/.pyenv/versions/3.7.4/bin/python3
> conda info --json

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

[svenefftinge]

Tested. I don't see the error message anymore.

[AlexTugarev]

@akosyakov should this be added to change logs?

[svenefftinge]

I wonder how declaration is different than definition... couldn't find any good information on the LSP spec.

[akosyakov]

code changes looks good to me

[akosyakov]

@svenefftinge It was added for cpp initially: microsoft/vscode#62483 Internet says: https://www.geeksforgeeks.org/difference-between-definition-and-declaration/