[FEATURE_REQUEST] Add an expression calculation

[WingedSeal]

$a = ($b + $c) - 5

[Nico314159]

Hmm, maybe we could have a multi-step rewrite for this? (just an idea I thought of for how the code would work)

So it would first turn that into

$a = $b + $c;
$a -= 5;

and then

$a = $b;
$a += $c;
$a -= 5;

and then turn it into mcfunction.