Feature request: Equivalence of expressions

A few equivalence understandings would help grading a lot:
- [x] it should treat “$b + $a” the same as “$a + $b” for binary operators that we specify
- [x] it should treat “$a += $b;” the same as “$a = $a + $b” for binary operators
- [ ] It should treat $a++ the same as $a += 1
- [ ] It should treat $a <= $b the same as $b >= $a
- [ ] It should treat $a < $b the same as $b > $a
- [ ] it should treat “var $a= $x;” the same as “var $a; $a=$x”

No files, tests, or entry points are named. Start by reviewing how the already-checked equivalence cases are represented and handled, then determine the scope and semantics of the remaining unchecked cases. Done means the requested expression pairs are recognized equivalently during grading, with coverage for each supported transformation.

Written by the indexing model from the issue text.