[vscode] support env variable substitution

[akosyakov]

What it does

support env:NAME variable substitution, see VS Code doc: https://code.visualstudio.com/docs/editor/variables-reference#_environment-variables

How to test

Modify launch configuration to reference some env variable and launch it. See for example how PATH env variable is referenced in Run Mocha Test configuration below, and then PATH value is printed to the debug console:

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

[azatsarynnyy]

I've tested the PR with several simple tasks, like the following:

    }
      "label": "test env var",
      "type": "shell",
      "command": "sleep 1 && echo ${env:USER}"
    }

Works well!

[akosyakov]

@azatsarynnyy Would you be fine if i assign you for reviews more often? We need help with testing and ensuring quality. Even half an hour couple days a week would help.

[azatsarynnyy]

@akosyakov sure, I don't mind 🙂
I'll be glad to help as much as possible for the common good.

[paul-marechal]

Works well :)

[AlexTugarev]

It works as advertised!
Tested with command and args.