The `is` operator was placed at `PREC_IS` which had lower precedence than `PREC_EQUALITY`, causing expressions like `true == 10 is Num` to parse incorrectly. Moved `PREC_IS` above `PREC_EQUALITY` in the precedence enum to give `is` higher binding power than `==` and `!=`. Added precedence tests in new `test/precedence.wren` file and removed TODO comments about precedence from `test/is/is.wren`.
