Implement ISUPPORT parsing + the UTF8ONLY IRCv3 specification

[progval]

ISUPPORT syntax: https://modern.ircdocs.horse/#rplisupport-005

Relevant bits from the UTF8ONLY specification:

Servers publishing this token MUST NOT relay content [...] containing non-UTF-8 data to clients

If a client implementing this specification sees this token, they MUST set their outgoing encoding to UTF-8 without requiring any user intervention.

This has the following effects:

• With the default configuration, this does nothing because Srain uses UTF-8 by default
• If a non-UTF-8 encoding is specified, it overrides the user's configuration if the server advertises UTF8ONLY.

This seems to affect only the incoming encoding, because Srain always send UTF-8 regardless of how I configured it. (Which is nice IMO, but it looks like a bug.)