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/implicit_receiver/nested_class.wren

@@ -3,7 +3,7 @@ class Outer {
     IO.print("outer getter")
   }
 
-  setter = value {
+  setter=(value) {
     IO.print("outer setter")
   }
 
@@ -21,7 +21,7 @@ class Outer {
         IO.print("inner getter")
       }
 
-      setter = value {
+      setter=(value) {
         IO.print("inner setter")
       }