Provide autocompletion for variables tox.ini files

[pkirch]

Problem

In larger tox.ini files many variables are used. In order to ease development it would be quite helpful to have autocompletion for variables while development.

Solution

• As soon the user is typing { inside a tox.ini file in the VS Code editor the autocompletion feature should provide a list of valid environment variables defined in the tox.ini file. Example of a custom variable: {env:MY_SETTING}
• Further it would be helpful also to have Tox's integrated variables available in the autocomplete feature. Example: {toxinidir}

Related

This feature could build upon a common foundation which is already used for issue #19.