Add loading extension loading indicator to status bar

[qubitron]

We are moving most of the extension loading out of the activation path, so there will be a period of time after activation to when the extension is ready to start.

While the Python extension is loading in the background it should say "Python extension loading..." in the leftmost position of the status bar, with a spinning icon.

Loading the extension includes:

• Calculating interpreters and environments available
• Validating the users setup
• Starting the language server