Creating new cell in Notebook should alternate on the type of cell

[NicolasAnquetil]

Currently creating a new cell uses the same type of the current cell:
If we are in a microdown cell, it creates a microdown cell, if we are in a code cell, it creates a code cell

It seems logical that if the user creates a new cell, they want something different than what they have.
So:

• when in a text cell, creating a new cell should give a code cell
• when in a code cell, creating a new cell should give a text cell