fix: correct operator precedence for % token from PREC_TERM to PREC_FACTOR

The modulo operator was incorrectly assigned PREC_TERM precedence, making it
evaluate at the same level as + and -. This caused expressions like "13 + 1 % 7"
to parse as "(13 + 1) % 7" instead of the correct "13 + (1 % 7)".

Changed the grammar rule for TOKEN_PERCENT to use PREC_FACTOR, matching the
precedence of * and / operators. Added a precedence test case in mod.wren to
verify the fix.

test/number/mod.wren

@@ -10,5 +10,8 @@ IO.print(-4.2 % -3.1) // expect: -1.1
 // Left associative.
 IO.print(13 % 7 % 4) // expect: 2
 
+// Precedence.
+IO.print(13 + 1 % 7) // expect: 14
+
 // TODO: Unsupported RHS types.
 // TODO: Error on mod by zero.