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.

src/wren_core.c

@@ -77,9 +77,9 @@ static const char* libSource =
 "    return result\n"
 "  }\n"
 "\n"
-"  + that {\n"
+"  +(other) {\n"
 "    var result = this[0..-1]\n"
-"    for (element in that) {\n"
+"    for (element in other) {\n"
 "      result.add(element)\n"
 "    }\n"
 "    return result\n"