[commands] Add initial implementation of hybrid commands

Hybrid commands allow a regular command to also double as a slash
command, assuming it meets the subset required to function.

docs/ext/commands/api.rst

@@ -114,6 +114,13 @@ Decorators
 .. autofunction:: discord.ext.commands.group
     :decorator:
 
+.. autofunction:: discord.ext.commands.hybrid_command
+    :decorator:
+
+.. autofunction:: discord.ext.commands.hybrid_group
+    :decorator:
+
+
 Command
 ~~~~~~~~~
 
@@ -173,6 +180,51 @@ GroupMixin
     .. automethod:: GroupMixin.group(*args, **kwargs)
         :decorator:
 
+HybridCommand
+~~~~~~~~~~~~~~
+
+.. attributetable:: discord.ext.commands.HybridCommand
+
+.. autoclass:: discord.ext.commands.HybridCommand
+    :members:
+    :special-members: __call__
+    :exclude-members: after_invoke, before_invoke, error
+
+    .. automethod:: HybridCommand.after_invoke()
+        :decorator:
+
+    .. automethod:: HybridCommand.before_invoke()
+        :decorator:
+
+    .. automethod:: HybridCommand.error()
+        :decorator:
+
+HybridGroup
+~~~~~~~~~~~~
+
+.. attributetable:: discord.ext.commands.HybridGroup
+
+.. autoclass:: discord.ext.commands.HybridGroup
+    :members:
+    :inherited-members:
+    :exclude-members: after_invoke, before_invoke, command, error, group
+
+    .. automethod:: HybridGroup.after_invoke()
+        :decorator:
+
+    .. automethod:: HybridGroup.before_invoke()
+        :decorator:
+
+    .. automethod:: HybridGroup.command(*args, **kwargs)
+        :decorator:
+
+    .. automethod:: HybridGroup.error()
+        :decorator:
+
+    .. automethod:: HybridGroup.group(*args, **kwargs)
+        :decorator:
+
+
 .. _ext_commands_api_cogs:
 
 Cogs
@@ -631,6 +683,9 @@ Exceptions
 .. autoexception:: discord.ext.commands.CommandRegistrationError
     :members:
 
+.. autoexception:: discord.ext.commands.HybridCommandError
+    :members:
+
 
 Exception Hierarchy
 ~~~~~~~~~~~~~~~~~~~~~
@@ -687,6 +742,7 @@ Exception Hierarchy
             - :exc:`~.commands.CommandInvokeError`
             - :exc:`~.commands.CommandOnCooldown`
             - :exc:`~.commands.MaxConcurrencyReached`
+            - :exc:`~.commands.HybridCommandError`
         - :exc:`~.commands.ExtensionError`
             - :exc:`~.commands.ExtensionAlreadyLoaded`
             - :exc:`~.commands.ExtensionNotLoaded`