commands: change API of "make_seed" and "create" commands

instead of "segwit" boolean, take a "seed_type" optional arg
default seed_type to "segwit"
previously these commands created legacy seeds by defalt

electrum/version.py

@@ -19,3 +19,4 @@ def seed_prefix(seed_type):
         return SEED_PREFIX_2FA
     elif seed_type == '2fa_segwit':
         return SEED_PREFIX_2FA_SW
+    raise Exception(f"unknown seed_type: {seed_type}")