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.

builtin/core.wren

@@ -34,9 +34,9 @@ class List is Sequence {
     return result
   }
 
-  + that {
+  +(other) {
     var result = this[0..-1]
-    for (element in that) {
+    for (element in other) {
       result.add(element)
     }
     return result