Refactor how commands are registered

[cunla]

Fix #78.

Now there is support for sub-commands.

[github-actions]

Coverage report:

Name                                              Stmts   Miss Branch BrPart  Cover
-----------------------------------------------------------------------------------
fakeredis/__init__.py                                 6      2      0      0    67%
fakeredis/_basefakesocket.py                        225     12     96     11    92%
fakeredis/_commands.py                              274      6     94      3    97%
fakeredis/_fakesocket.py                             16      0      0      0   100%
fakeredis/_helpers.py                               166      4     61      3    97%
fakeredis/_msgs.py                                   52      0      0      0   100%
fakeredis/_server.py                                122      6     42      5    93%
fakeredis/_zset.py                                   64      0     16      0   100%
fakeredis/aioredis.py                               153      7     43      5    94%
fakeredis/commands_mixins/__init__.py                 0      0      0      0   100%
fakeredis/commands_mixins/bitmap_mixin.py            44      1      8      1    96%
fakeredis/commands_mixins/connection_mixin.py        21      0      4      0   100%
fakeredis/commands_mixins/generic_mixin.py          258     17    120     10    92%
fakeredis/commands_mixins/hash_mixin.py              82      1     16      1    98%
fakeredis/commands_mixins/list_mixin.py             176      5     76      5    96%
fakeredis/commands_mixins/pubsub_mixin.py            60      0     24      5    94%
fakeredis/commands_mixins/scripting_mixin.py        143      4     68      4    96%
fakeredis/commands_mixins/server_mixin.py            49      0     14      0   100%
fakeredis/commands_mixins/set_mixin.py              139     12     58      6    88%
fakeredis/commands_mixins/sortedset_mixin.py        320      8    137      7    97%
fakeredis/commands_mixins/string_mixin.py           162      3     64      3    97%
fakeredis/commands_mixins/transactions_mixin.py      71      2     20      1    97%
-----------------------------------------------------------------------------------
TOTAL                                              2603     90    961     70    95%