[commands] Add support for FlagConverter in hybrid commands

This works by unpacking and repacking the flag arguments in a flag.
If an unsupported type annotation is found then it will error at
definition time.

discord/ext/commands/flags.py

@@ -28,7 +28,7 @@ import inspect
 import re
 import sys
 from dataclasses import dataclass, field
-from typing import TYPE_CHECKING, Any, Dict, Iterator, List, Literal, Optional, Pattern, Set, Tuple, Union
+from typing import TYPE_CHECKING, Any, Dict, Iterator, List, Literal, Optional, Pattern, Set, Tuple, Type, Union
 
 from discord.utils import MISSING, maybe_coroutine, resolve_annotation
 
@@ -44,7 +44,7 @@ __all__ = (
 
 
 if TYPE_CHECKING:
-    from typing_extensions import Self
+    from typing_extensions import Self, TypeGuard
 
     from ._types import BotT
     from .context import Context
@@ -76,6 +76,9 @@ class Flag:
         A negative value indicates an unlimited amount of arguments.
     override: :class:`bool`
         Whether multiple given values overrides the previous value.
+    description: :class:`str`
+        The description of the flag. Shown for hybrid commands when they're
+        used as application commands.
     """
 
     name: str = MISSING
@@ -85,6 +88,7 @@ class Flag:
     default: Any = MISSING
     max_args: int = MISSING
     override: bool = MISSING
+    description: str = MISSING
     cast_to_dict: bool = False
 
     @property
@@ -104,6 +108,7 @@ def flag(
     max_args: int = MISSING,
     override: bool = MISSING,
     converter: Any = MISSING,
+    description: str = MISSING,
 ) -> Any:
     """Override default functionality and parameters of the underlying :class:`FlagConverter`
     class attributes.
@@ -128,8 +133,23 @@ def flag(
     converter: Any
         The converter to use for this flag. This replaces the annotation at
         runtime which is transparent to type checkers.
+    description: :class:`str`
+        The description of the flag. Shown for hybrid commands when they're
+        used as application commands.
     """
-    return Flag(name=name, aliases=aliases, default=default, max_args=max_args, override=override, annotation=converter)
+    return Flag(
+        name=name,
+        aliases=aliases,
+        default=default,
+        max_args=max_args,
+        override=override,
+        annotation=converter,
+        description=description,
+    )
+
+
+def is_flag(obj: Any) -> TypeGuard[Type[FlagConverter]]:
+    return hasattr(obj, '__commands_is_flag__')
 
 
 def validate_flag_name(name: str, forbidden: Set[str]) -> None: