Detect ActiveState Python Runtimes

[mitchell-as]

The ActiveState Platform provides downloadable Python runtimes that can be installed on end-user machines with the help of a command line tool. VSCode can leverage this command line tool to find these Python runtimes and allow the user to set them as the default Python interpreter and/or switch between them using either the "Python: Select Interpreter" command, or the statusbar button.

ActiveState will supply the PR for this feature.

[karrtikr]

Thanks for the PR, we'll have a look and decide accordingly.