feat: allow trailing comma in list literal by early break on right bracket

Add support for trailing commas in list literals by checking for TOKEN_RIGHT_BRACKET
after parsing the last element in the list() function. When a trailing comma is
followed by the closing bracket, the loop breaks immediately instead of attempting
to parse another element.

Include a test case verifying that lists with trailing commas parse correctly and
elements are accessible by index.

test/language/list/trailing_comma.wren

@@ -0,0 +1,7 @@
+var list = [
+  "a",
+  "b",
+]
+
+IO.print(list[0]) // expect: a
+IO.print(list[1]) // expect: b