feat: add conditional operator and fn toString to wren compiler and core

Add TOKEN_QUESTION token type and implement conditional() parsing function in wren_compiler.c to support the ternary conditional operator (condition ? then : else). Register fn_toString native in wren_core.c to return "<fn>" string for function objects. Include comprehensive test suite covering precedence, short-circuit evaluation, and error cases for missing tokens.

test/conditional/precedence.wren

@@ -0,0 +1,59 @@
+class Foo {
+  static bar { return true }
+  static baz { return 1 }
+}
+
+// Condition precedence.
+IO.print(true ? 1 : 2) // expect: 1
+IO.print((true) ? 1 : 2) // expect: 1
+IO.print([true][0] ? 1 : 2) // expect: 1
+IO.print(Foo.bar ? 1 : 2) // expect: 1
+IO.print(3..4 ? 1 : 2) // expect: 1
+IO.print(3 * 4 ? 1 : 2) // expect: 1
+IO.print(3 + 4 ? 1 : 2) // expect: 1
+IO.print(true || false ? 1 : 2) // expect: 1
+IO.print(!false ? 1 : 2) // expect: 1
+IO.print(~0 ? 1 : 2) // expect: 1
+IO.print(3 is Num ? 1 : 2) // expect: 1
+IO.print(new Foo ? 1 : 2) // expect: 1
+IO.print(fn true ? 1 : 2) // expect: <fn>
+
+var a = 0
+IO.print(a = 3 ? 1 : 2) // expect: 1
+IO.print(a) // expect: 1
+
+// Then branch precedence.
+IO.print(true ? (1) : 2) // expect: 1
+IO.print(true ? [1][0] : 2) // expect: 1
+IO.print(true ? Foo.baz : 2) // expect: 1
+IO.print(true ? 3..4 : 2) // expect: 3..4
+IO.print(true ? 3 * 4 : 2) // expect: 12
+IO.print(true ? 3 + 4 : 2) // expect: 7
+IO.print(true ? 1 || false : 2) // expect: 1
+IO.print(true ? !true : 2) // expect: false
+IO.print(true ? ~0 : 2) // expect: 4294967295
+IO.print(true ? 3 is Bool : 2) // expect: false
+IO.print(true ? new Foo : 2) // expect: instance of Foo
+// TODO: Is this what we want?
+IO.print(true ? fn 1 : 2) // expect: <fn>
+
+IO.print(true ? a = 5 : 2) // expect: 5
+IO.print(a) // expect: 5
+
+// Else branch precedence.
+IO.print(false ? 1 : (2)) // expect: 2
+IO.print(false ? 1 : [2][0]) // expect: 2
+IO.print(false ? 2 : Foo.baz) // expect: 1
+IO.print(false ? 1 : 3..4) // expect: 3..4
+IO.print(false ? 1 : 3 * 4) // expect: 12
+IO.print(false ? 1 : 3 + 4) // expect: 7
+IO.print(false ? 1 : 2 || false) // expect: 2
+IO.print(false ? 1 : !false) // expect: true
+IO.print(false ? 1 : ~0) // expect: 4294967295
+IO.print(false ? 1 : 3 is Num) // expect: true
+IO.print(false ? 1 : new Foo) // expect: instance of Foo
+IO.print(false ? 1 : fn 2) // expect: <fn>
+
+// Associativity.
+IO.print(true ? 2 : true ? 4 : 5) // expect: 2
+IO.print(false ? 2 : true ? 4 : 5) // expect: 4