Make scheme part of parse_url comply with rfc3986

[AkameOuO]

According to the rfc3986 Section 3.1:

Scheme names consist of a sequence of characters beginning with a
letter and followed by any combination of letters, digits, plus
("+"), period ("."), or hyphen ("-").

parse_url currently accepts schemes that start with non-alphabetic characters.

[iluuu1994]

See https://wiki.php.net/rfc/url_parsing_api. parser_url is heavily used for invalid URLs, and past changes have lead to unintended BC breaks (which you can see in your own PR). Instead, we're planning to add RFC3986 (and WHATWG) compliant parsers that do not accept invalid URLs.

[AkameOuO]

I see. Sorry I didn't notice that

[iluuu1994]

No worries! Thanks for your PR nonetheless!