Fix midpoint expression to use converted expressions

[Dev-Xeiji]

Problem

The midpoint expression only worked properly when both inputs were already locations or vectors.
#8625

Solution

The expression now checks the possible return types using existing converters, then converts both inputs to the correct type during initialization. This allows values like blocks to be treated as locations when a valid converter exists.

PS: The actual midpoint calculation was left unchanged.

Testing Completed

Tested manually with two defined variables and confirmed that the midpoint expression now works when the blocks are converted to locations.

on load:
    set {-a} to block at location(0,0,0)
    set {-b} to block at location(10,10,10)
    broadcast midpoint of {-a} and {-b}

Supporting Information

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Completes: #8625
Related: none
AI assistance: none