WASIp3 request#set-scheme is more lenient than RFC 9110

[wingo]

RFC 3986 §3.1, referred to by RFC 9110, specifies that URI schemes should begin with an ascii alphabetic character and then continue the same but also allowing +, -, numbers and .:

      scheme      = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )

And in Wasmtime, we have the expected behavior in that request.set_method allows the productions above and rejects most others, but it is more lenient in that it allows schemes that begin with a character in +-.0123456789~. Probably that needs tightening up.

[wingo]

We should probably also eagerly lower-case "other" schemes, i.e. that get-method after set-method("A") should return "a":

Although schemes are case-insensitive, the canonical form is lowercase and documents that specify schemes must do so with lowercase letters. An implementation should accept uppercase letters as equivalent to lowercase in scheme names e.g., allow "HTTP" as well as "http") for the sake of robustness but should only produce lowercase scheme names for consistency.