fix: raise precedence of is above equality operators in parser

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`.

test/precedence.wren

@@ -0,0 +1,41 @@
+// * has higher precedence than +.
+IO.print(2 + 3 * 4) // expect: 14
+
+// * has higher precedence than -.
+IO.print(20 - 3 * 4) // expect: 8
+
+// / has higher precedence than +.
+IO.print(2 + 6 / 3) // expect: 4
+
+// / has higher precedence than -.
+IO.print(2 - 6 / 3) // expect: 0
+
+// < has higher precedence than ==.
+IO.print(false == 2 < 1) // expect: true
+
+// > has higher precedence than ==.
+IO.print(false == 1 > 2) // expect: true
+
+// <= has higher precedence than ==.
+IO.print(false == 2 <= 1) // expect: true
+
+// >= has higher precedence than ==.
+IO.print(false == 1 >= 2) // expect: true
+
+// is has higher precedence than ==.
+IO.print(true == 10 is Num) // expect: true
+IO.print(10 is Num == false) // expect: false
+
+// Unary - has lower precedence than ..
+IO.print(-"abc".count) // expect: -3
+
+// 1 - 1 is not space-sensitive.
+IO.print(1 - 1) // expect: 0
+IO.print(1 -1)  // expect: 0
+IO.print(1- 1)  // expect: 0
+IO.print(1-1)   // expect: 0
+
+// TODO: %, associativity.
+
+// Using () for grouping.
+IO.print((2 * (6 - (2 + 2)))) // expect: 4