feat: rename List.new(_,_) to List.filled(_,_) for clarity

The two-argument constructor `List.new(size, element)` was renamed to
`List.filled(size, element)` to better convey its purpose of creating a
list filled with a default value. The underlying primitive
`list_newWithSizeDefault` remains unchanged. The test file
`test/core/list/new_extra_args.wren` was updated to use the new name.

test/core/list/new_extra_args.wren

@@ -3,6 +3,6 @@ var list = List.new(5)
 System.print(list.count) // expect: 5
 System.print(list) // expect: [null, null, null, null, null]
 
-var list2 = List.new(5, 2)
+var list2 = List.filled(5, 2)
 System.print(list2.count) // expect: 5
 System.print(list2) // expect: [2, 2, 2, 2, 2]