fix: correct CHANMODES ISUPPORT classification and deduplicate MintChannelHashcash

- Move H from Type B to Type C in CHANMODES ISUPPORT string (H takes
  a parameter only when set, not when unset)
- Refactor MintChannelHashcash to delegate to hashcash.BodyHash() and
  hashcash.MintChannelStamp() instead of reimplementing them

internal/handlers/api.go

@@ -300,7 +300,7 @@ func (hdlr *Handlers) deliverWelcome(
 		[]string{
 			"CHANTYPES=#",
 			"NICKLEN=32",
-			"CHANMODES=,H,," + "imnst",
+			"CHANMODES=,,H," + "imnst",
 			"NETWORK=neoirc",
 			"CASEMAPPING=ascii",
 		},