Compiler: Allow local variable declarations within block

[FunctionPoint]

Example

myMethod
    [ :a1 :a2 | 
        | v | v := 1. self onValue: v ] value.
!

Pharo has this too.
Its is especially useful when blocks are called async,
that need the correct variable instance.

[FunctionPoint]

But... as a workaround you can just declare (extra) block arguments temp vars.
Those are not passed to the block and initialized to JS undefined by default (not nil !).