Extend for loops with custom index support

This patch was authored by @vzsg and released by @0xTim.

Traditionally, the for loops in Leaf declare three variables in the loop's local context: isFirst, isLast and index. The first two aren't particularly interesting, but being able to access all indices in a nested for loop situation is sometimes useful when working with two dimensions at once. Not to mention it just popped up on Discord earlier today.

This PR extends Loop with the option of renaming index.

Example syntax:

#for(i, array in arrays):
  #for(j, element in array):
    (#(i), #(j)): #(element)
  #endfor
#endfor

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Should be semver-minor.