feature: enhance expand_styles_prop to merge className and style props

[pedrobslisboa]

Description

This PR enables the styles to work with both the' className' and' style' attributes.

It also fixes an error on expanding Optional "styles", before it was validating the frt and snd values instead of the styles itself. Now, we use an Option.map to retrieve the correct values from the option content.

File "packages/server-reason-react-ppx/test/test.re", line 631, characters 18-24:
631 |   let div = <div ?styles />;
                        ^^^^^^
Error: This expression has type 'a option
       but an expression was expected of type 'b * 'c
Had 1 error, waiting for filesystem changes...     

(* Before *)
React.createElementWithKey ~key:None "div"
  (Stdlib.List.filter_map Stdlib.Fun.id
     [
       (match (fst x : string option) with
       | None -> None
       | Some v -> Some (React.JSX.String ("class", "className", v)));
       (match (snd x : ReactDOM.Style.t option) with
       | None -> None
       | Some v -> Some (React.JSX.Style v));
     ])
  []

(* After *)
React.createElementWithKey ~key:None "div"
  (Stdlib.List.filter_map Stdlib.Fun.id
     [
       (match (Option.map (fun x -> fst x) x : string option) with
       | None -> None
       | Some v -> Some (React.JSX.String ("class", "className", v)));
       (match (Option.map (fun x -> snd x) x : ReactDOM.Style.t option) with
       | None -> None
       | Some v -> Some (React.JSX.Style v));
     ])
  []

Examples:

// Expanding styles prop into className and style props
<div styles=x />;

// Expanding styles prop into className and style props with optional styles
<div styles=?x />;

// Expanding styles prop into className and style props with className already set
<div className="lola" styles=x />;

// Expanding styles prop into className and style props with style already set
<div style={ReactDOM.Style.make(~backgroundColor="gainsboro", ())} styles=x />;

// Expanding styles prop into className and style props with className and style already set
<div className="lola" style={ReactDOM.Style.make(~backgroundColor="gainsboro", ())} styles=x />;

// Expanding styles prop into className and style props with optional styles prop
<div styles=?x />;

// Expanding styles prop into className and style props with optional styles prop and className already set
<div className="lola" styles=?x />;

// Expanding styles prop into className and style props with optional styles prop and style already set
<div style={ReactDOM.Style.make(~backgroundColor="gainsboro", ())} styles=?x />;

// Expanding styles prop into className and style props with optional styles prop and className and style already set
<div className="lola" style={ReactDOM.Style.make(~backgroundColor="gainsboro", ())} styles=?x />;```