feat: add scheme start state

[anonrig]

No description provided.

[lemire]

Random comments which you may consider...

1. There is currently a mix of different styles. The most common in C++ (at least in the standard library) is all lower-case (except for macros) with underscores: void parse_state(). Whatever one does, we should not mix parseState() and parse_state() in the same code base.
2. As much as possible, one should try to initialize the variables in the following manner (for example):

parser::parser(const std::string_view input, std::optional<ada::url> optional_base_url, 
   std::optional<ada::encoding_type> encoding_override):  buffer{}, 
   pointer{input.begin()}, encoding{encoding_override.value_or(ada::encoding_type::UTF8)} {
...
}

That is, once you enter the curly bracket part of the constructor, your attributes should have a clear state. I think it is considered form to do the initialization in the curly-bracket component of the constructor.
3. In the header file, you can make sure the attributes of your class are always initialized with the curly-bracket trick:

    std::string_view input{};
    std::string buffer{};
    bool at_sign_seen{}; // will be false
    bool inside_brackets{};
    bool password_token_seen{};

3. Tools like clang-format will allow one to set a line limit. I think it is often considered bad form to have lines much longer than 80 characters.

[lemire]

These are not requests for changes. They are comments.

[anonrig]

Thank you for the review @lemire. Much appreciated. Let's merge this now, and I'll apply your comments in a separate pull request.