feat: require parentheses around operator and setter method parameters in Wren

Enforce syntactic requirement for parentheses in operator method signatures (e.g., `+(other)` instead of `+ other`) and setter definitions (e.g., `bar=(value)` instead of `bar = value`). Update the compiler's `infixSignature`, `mixedSignature`, and `namedSignature` functions to consume explicit `(` and `)` tokens around parameter names. Migrate all built-in core library operator definitions and test fixtures to the new parenthesized syntax.

test/setter/associativity.wren

@@ -1,7 +1,7 @@
 class Foo {
   new(value) { _value = value }
   toString { _value }
-  bar = value {
+  bar=(value) {
     _value = value
     return value
   }