Implement a least breaking approach to slash commands (#39)

* Most slash command support completed, needs some debugging (and reindent)

* Implement a ctx.send helper for slash commands

* Add group command support

* Add Option converter, fix default optional, fix help command

* Add client.setup and move readying commands to that

* Implement _FakeSlashMessage.from_interaction

* Rename normmal_command to message_command

* Add docs for added params

* Add slash_command_guilds to bot and decos

* Fix merge conflict

* Remove name from commands.Option, wasn't used

* Move slash command processing to BotBase.process_slash_commands

* Create slash_only.py

Basic example for slash commands

* Create slash_and_message.py

Basic example for mixed commands

* Fix slash_command and normal_command bools

* Add some basic error handling for registration

* Fixed converter upload errors

* Fix some logic and make an actual example

* Thanks Safety Jim

* docstrings, *args, and error changes

* Add proper literal support

* Add basic documentation on slash commands

* Fix non-slash command interactions

* Fix ctx.reply in slash command context

* Fix typing on Context.reply

* Fix multiple optional argument sorting

* Update ctx.message docs to mention error instead of warning

* Move slash command creation to BotBase

* Fix code style issues with Black

* Rearrange some stuff and add flag support

* Change some errors and fix interaction.channel fixing

* Fix slash command quoting for *args

Co-authored-by: iDutchy <42503862+iDutchy@users.noreply.github.com>
Co-authored-by: Lint Action <lint-action@samuelmeuli.com>

discord/ext/commands/errors.py

@@ -33,6 +33,7 @@ if TYPE_CHECKING:
 
     from .converter import Converter
     from .context import Context
+    from .core import Command
     from .cooldowns import Cooldown, BucketType
     from .flags import Flag
     from discord.abc import GuildChannel
@@ -93,6 +94,7 @@ __all__ = (
     "ExtensionFailed",
     "ExtensionNotFound",
     "CommandRegistrationError",
+    "ApplicationCommandRegistrationError",
     "FlagError",
     "BadFlagArgument",
     "MissingFlagArgument",
@@ -1014,6 +1016,25 @@ class CommandRegistrationError(ClientException):
         super().__init__(f"The {type_} {name} is already an existing command or alias.")
 
 
+class ApplicationCommandRegistrationError(ClientException):
+    """An exception raised when a command cannot be converted to an
+    application command.
+
+    This inherits from :exc:`discord.ClientException`
+
+    .. versionadded:: 2.0
+
+    Attributes
+    ----------
+    command: :class:`Command`
+        The command that failed to be converted.
+    """
+
+    def __init__(self, command: Command, msg: str = None) -> None:
+        self.command = command
+        super().__init__(msg or f"{command.qualified_name} failed to converted to an application command.")
+
+
 class FlagError(BadArgument):
     """The base exception type for all flag parsing related errors.