be able to initialize lists using the assign operator

[MajeedKazemi]

• instead of relying on clearing a list and using the add block to add items to a list, the toolbox will be updated at some point to something like this:
• however, this is not good as it doesn't allow entering complex items (e.g. defining a list with 3 random numbers)
• pxt-blockly has a + button used in many blocks such as the join operator, we should use that to add new holes and the commas between them (a similar behavior happens in CodeStruct)