Added pre de/increment operators. Fixed post de/increment operators.

- [ This commit is part of a larger script engine related update ]
- Suffix ++ and -- operators now behave like in other languages
  (updating the variable *after* its value is returned.)
- Prefix ++ and -- operators are added for parity with other
  scripting/programming languages. They update the variable they're
  applied to *before* returning its value.
- Please note that the implementation of the prefix form of those
  operators (like it happens in most languages) is more efficient than
  the suffix form. '++.@i' is (slightly) faster than '.@i++', or at
  least not slower.
- Fixed some outdated script debug functions.
- Follow-up to c18f438.

Signed-off-by: Haru <haru@dotalux.com>