Add implementation signature of parse function to fix typings

[vkrol]

Without implementation signature we get an error when passing string | RegExp to the parse function:

declare const route: string | RegExp;
parse(route);
/*
No overload matches this call.
  Overload 1 of 2, '(route: string, loose?: boolean | undefined): { keys: string[]; pattern: RegExp; }', gave the following error.
    Argument of type 'string | RegExp' is not assignable to parameter of type 'string'.
      Type 'RegExp' is not assignable to type 'string'.
  Overload 2 of 2, '(route: RegExp): { keys: false; pattern: RegExp; }', gave the following error.
    Argument of type 'string | RegExp' is not assignable to parameter of type 'RegExp'.
      Type 'string' is not assignable to type 'RegExp'.(2769)
*/

https://www.typescriptlang.org/play/?#code/CYUwxgNghgTiAEAzArgOzAFwJYHtXwAdYBnEAChh2QxAC55iMYtUBzAGnghx1IH56AIx4QQUVAEp6AbwBQASADWIAJ7F6jZmwDaAXQDcCohhoxU9AEohWAUQAeBQwF9Zs0JFgIU6bHkIlySmo6eCtbByl4OSVVdSQoCFJDeWNTc1Dre0dZF1d3aDh4MDxGeCCaDSYWVngAHwzw7NkiGFIKKhoJQyA

To fix this, I add the implementation signature to the parse function: https://www.typescriptlang.org/play/?ssl=17&ssc=14&pln=15&pc=1#code/CYUwxgNghgTiAEAzArgOzAFwJYHtXwAdYBnEAChh2QxAC55iMYtUBzAGnghx1IH56AIx4QQUVAEp6AbwBQASADWIAJ7F6jZmwDaAXQDcCohhoxU9AEohWAUQAeBQwF9ZoSLAQp02PIRLlKajp4K1sHKXg5JVV1JCgIUkN5Y1NzEOt7R1kXN2g4JDRMXHwiGFIKKhoNJhZWeAAfdLCCTm5eEAF4YRxRcQio5TVqrVY9BriEkCSUkDNLDIdnWWXcj3gwPEZ4QKqGGrZx0MzDWVLynZAJfSA