Don't use generic function for finding method duplicates and add tests

test/language/method/duplicate_methods.wren

@@ -0,0 +1,11 @@
+class Foo {
+  construct new() {}
+  foo() {}
+  foo() {} // expect error
+  
+  static bar() {}
+  static bar() {} // expect error
+  
+  baz() {}
+  static baz() {}
+}