add a better support for loops in incremental interpreter

[jimy-byerley]

Right now, the support for loops is simply to ignore their variables and not place any backup step inside. Forcing us to write the loops the old way, by writing it completely before being able to test it.

A better support could take inspiration from function edition:

• the shortcut to edit the function will edit the loop when the cursor is in the area of the loop's block
• the edition will freeze the loop initial context
• for for loops it will allow to change between elements of the iterator to test the loop body
• the loop body will be edited in its proper interactive interpreter like functions body