Add fast path to pathname start state

[anonrig]

This particular change adds a fast-path for implementors. We already use this in Ada (available at https://github.com/ada-url/ada/blob/main/src/parser.cpp#L650).

This change removes the performance penalty of URLs ending with /. As someone can imagine, it's pretty common for a user to send a request to https://ada-url.com/.

This particular change improved the performance of Ada ~9-10%. ada-url/ada#59

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[rmisev]

1. As the title says, this optimization must be in path start state, but you added it to relative slash state; by mistake?
2. I think this optimization can be applied if c is /, \ or EOF, not just /. After all, http://h/, http://h\ and http://h are parsed to http://h/.
3. I guess the added step must return url, not output.