runtime(Js.Re): port @mel.send externals to use @mel.this (#1307)

jscomp/runtime/js_re.ml

@@ -154,7 +154,7 @@ external unicode : t -> bool = "unicode"
 [@@mel.get]
 (** returns a bool indicating whether the [unicode] flag is set *)
 
-external exec : str:string -> t -> result option = "exec"
+external exec : str:string -> (t[@mel.this]) -> result option = "exec"
 [@@mel.send] [@@mel.return null_to_opt]
 (** executes a search on a given string using the given RegExp object
 
@@ -173,7 +173,7 @@ let result = re |. Js.Re.exec ~str:"The Quick Brown Fox Jumps Over The Lazy Dog"
 @see <https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/exec> MDN
 *)
 
-external test : str:string -> t -> bool = "test"
+external test : str:string -> (t[@mel.this]) -> bool = "test"
 [@@mel.send]
 (** tests whether the given RegExp object will match a given string