Don't let variables of different types have the same names.

[rachel-fenichel]

Scratch allows this for backwards compatibility, so we have the infrastructure to support it in blockly. But the default behaviour should be to require all variable names to be unique.

[moniika]

Corner case with creating variables in functions needs addressed.

[RoboErikG]

Also marking low priority since we haven't fully fleshed out how typed variables work with functions.

[BeksOmega]

I think this is related to #3877