Fixes the error message for #557. By not using the Ppxlib.Ast_pattern way and allow any expression in the pattern, and latter pattern match the right AST. This might cause some future maintainance in the future, but it's worth to have proper education to errors.

  File "input.re", line 3, characters 23-25:
  3 | let invalid_css = [%cx []];
                             ^^
  Error: [%cx] expects either a string of CSS or an array of CSS rules. 
  
  Example:
    [%cx "display: block"]
    [%cx [|CSS.display(`block)|]]
  
  More info: https://styled-ppx.vercel.app/reference/cx
  [1]

• Added the possibility to print examples by language (ocaml will print some ocaml syntax, etc)
• Ensure Error has a proper interface, created mli and improved the current fns