Upgrade to Blockly v12

[Jaqster]

For keyboard nav -
microsoft/pxt#10464

[microbit-matt-hillsdon]

We've started to explore this here in order to be able to use the latest Blockly keyboard plugin with MakeCode. But really keen to get to the point where this is done by the MakeCode team as I think you'll have the best idea of what to check/review. Feel free to pull in our diff if useful. There are for sure some styling issues (selection, disabled blocks, context menu, buttons in flyout - variables/functions).

[abchatra]

Updated to blockly 12.