feat: extract argument list parsing into finishArgumentList helper

Extract the duplicated argument list parsing logic from methodCall and
subscript into a shared finishArgumentList function. This unifies the
comma-separated expression parsing, arity tracking, and newline handling
into a single location. Also add a new test case verifying that newlines
after commas and before closing delimiters are correctly handled in both
method calls and subscript expressions.

test/method/newlines.wren

@@ -0,0 +1,20 @@
+class Foo {
+  method(a, b) { "method " + a + " " + b }
+  [a, b] { "subscript " + a + " " + b }
+}
+
+var foo = new Foo
+
+// Allow newlines after commas and before ")".
+IO.print(foo.method("a",
+
+    "b"
+
+    )) // expect: method a b
+
+// Allow newlines after commas and before "]".
+IO.print(foo["a",
+
+    "b"
+
+    ]) // expect: subscript a b