Added subtle hover effect for toast-close and Changed comment style to be able to minify files

[GeorgeFlorian]

I thought of adding a hover effect for toast-close class (the X sign on the toast) to make it more visible on Desktop:

.toast-close:hover {
    opacity: 1;
}

Also, since I am using this library on an ESP and I believe I am not the only one, saving memory is of utmost importance so I've changed the comment style from // comment to /* comment */ to be able to minify the files. Also added ; were it was missing.

Thank you.

[elie-g]

@GeorgeFlorian
You changed the indentation size. I'm not the owner nor am I a contributor to this repo but you should usually never change the indentation size in a pull request.
Doing so changes almost all lines which will most likely cause unecessary conflicts when merged with other PRs.
Moreover, it also makes it harder to see the real code differences (not spaces) and to find what modifications you made to the code.