[Commands] Refactor command and group decorators (#1818)

* [V3 Commands] Refactor command and group decorators

* Add some tests

* Fix docs reference

* Tweak Group's MRO

tests/core/test_commands.py

@@ -0,0 +1,34 @@
+import pytest
+from redbot.core import commands
+
+
+@pytest.fixture(scope="session")
+def group():
+    @commands.group()
+    async def fixturegroup(*args, **kwargs):
+        return args, kwargs
+
+    return fixturegroup
+
+
+def is_Command(obj):
+    return isinstance(obj, commands.Command)
+
+
+def is_Group(obj):
+    return isinstance(obj, commands.Group)
+
+
+def test_command_decorators(coroutine):
+    assert is_Command(commands.command(name="cmd")(coroutine))
+    assert is_Group(commands.group(name="grp")(coroutine))
+
+
+def test_group_decorator_methods(group, coroutine):
+    assert is_Command(group.command(name="cmd")(coroutine))
+    assert is_Group(group.group(name="grp")(coroutine))
+
+
+def test_bot_decorator_methods(red, coroutine):
+    assert is_Command(red.command(name="cmd")(coroutine))
+    assert is_Group(red.group(name="grp")(coroutine))