Drop usage of `mb_string`, deprecating related properties/methods

[oliverklee]

From #454:

I think we can just forget about parsing in multibyte charsets. For single-byte charsets, if we encounter a @charset declaration (or if the charset in the settings is anything other than UTF-8), we just convert to UTF-8 using iconv and re-start parsing.
This way, we can use the regular string functions everywhere and no longer need mbstring at all.

Also mark the corresponding configuration methods/properties as @deprecated #464 will be removed in version 10.