Create a mechanism and documentation for modes.

[ntoll]

Mu has modes. We need to reproduce the existing ones for:

• Adafruit
• Microbit
• PyGameZero
• Python3

Furthermore, we should make it easy for others to add further modes:

• ESP mode
• Web mode
• Lego mode
• ???

We should make it easy for folks to be able to publish their modes so others can use them.