ppx: fix inexistent locations on comps that return element list

[jchavarri]

Fixes #90.

Instead of threading locations to the make function that is ultimately passed to createElement (which is what was originally triggering the error) we wrap the result of "ml_component" (the implementation provided by user) with : React.element annotation.

Then, the error happens earlier, in code that is not generated by ppx but written by user.

Tested manually (idk how to test ppx with full typed compilation 😅 ):

File "example/src/HelloWorldOCaml.ml", line 5, characters 26-38:
5 | let%component make ~foo = [React.null]
                              ^^^^^^^^^^^^
Error: This expression has type 'a list
       but an expression was expected of type React.element

File "example/src/HelloWorldOCaml.ml", line 5, characters 24-36:
5 | let%component make () = [React.null]
                            ^^^^^^^^^^^^
Error: This expression has type 'a list
       but an expression was expected of type React.element