chathistory: Advertize MSGREFTYPES ISUPPORT token (#251)

https://ircv3.net/specs/extensions/chathistory#isupport-tokens

The spec says they should be 'in order of decreasing preference'.
As currently the only backend is in-memory, this doesn't matter so I
picked `msgid` first (as it's less ambiguous); but this can be revisited
later if/when adding a backend which is more efficient with timestamps.

src/modules/chathistory.c

@@ -48,6 +48,7 @@ MOD_INIT()
 MOD_LOAD()
 {
 	ISupportSetFmt(modinfo->handle, "CHATHISTORY", "%d", CHATHISTORY_LIMIT);
+	ISupportSetFmt(modinfo->handle, "MSGREFTYPES", "msgid,timestamp");
 	return MOD_SUCCESS;
 }