introduce `for i in range()` syntax

[MajeedKazemi]

a few things to think about for the for block:

• how will the iterator variable be accessible to the user? do they need to define it themselves? will something automatically be added to the variables section (or maybe right next to the for loop in the toolbox)? or when dropping for loop in the canvas, we should prompt the user and ask them what the variable should be?
• how many transition steps do we want here? range( ) to be included? or to be in the toolbox as a separate block?

[carlkma]

a lot of open-ended questions to consider! will certainly think about them, thanks!