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

[RFC 3986 §3.1](https://www.rfc-editor.org/rfc/rfc3986#section-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.