Create a VS Code extension for Darker

[akaihola]

In case you're interested in having your own VS Code extension, https://code.visualstudio.com/api/advanced-topics/python-extension-template should make it easy (as in "an afternoon or less" easy) to create an extension which ships with Darker in-box and that you control (see the Black and isort extensions as examples).

Originally posted by @brettcannon in #409

[brettcannon]

/cc @karthiknadig