feat: add user-defined operator methods with signature compilation

Add support for user-defined operator methods in classes by introducing a `SignatureFn` field to `GrammarRule` and implementing four signature compilation functions: `namedSignature`, `infixSignature`, `unarySignature`, and `mixedSignature`. These functions handle parameter parsing and name mangling for different operator types (infix, prefix, and mixed unary/infix). The change includes a new test file `test/method_operators.wren` that validates all supported operator overloads including arithmetic, comparison, and prefix operators.

test/method_operators.wren

@@ -0,0 +1,31 @@
+class Foo {
+  + other { "infix + " + other }
+  - other { "infix - " + other }
+  * other { "infix * " + other }
+  / other { "infix / " + other }
+  % other { "infix % " + other }
+  < other { "infix < " + other }
+  > other { "infix > " + other }
+  <= other { "infix <= " + other }
+  >= other { "infix >= " + other }
+  == other { "infix == " + other }
+  != other { "infix != " + other }
+
+  ! { "prefix !" }
+  - { "prefix -" }
+}
+
+var foo = Foo.new
+io.write(foo + "a") // expect: infix + a
+io.write(foo - "a") // expect: infix - a
+io.write(foo * "a") // expect: infix * a
+io.write(foo / "a") // expect: infix / a
+io.write(foo % "a") // expect: infix % a
+io.write(foo < "a") // expect: infix < a
+io.write(foo > "a") // expect: infix > a
+io.write(foo <= "a") // expect: infix <= a
+io.write(foo >= "a") // expect: infix >= a
+io.write(foo == "a") // expect: infix == a
+io.write(foo != "a") // expect: infix != a
+io.write(!foo) // expect: prefix !
+io.write(-foo) // expect: prefix -