Check: `can_redact`

Users have to implement manually a check to verify if a user performing a command can redact messages. Since it's a common pattern, we should implement a check for that in `checks.py`.

### Proposed API
```python
def can_redact() -> Callable:
async def _can_redact(ctx: Context) -> bool:
...

def wrapper(cmd: Command) -> Command:
cmd.check(_can_redact)
```

**Example**
```python
from matrix.checks import is_admin

@can_redact()
@bot.command()
async def kick(ctx, user: str) -> None:
```

### Before opening a PR
- Write unit tests
- Run `mypy`
- Run `pytest tests/`
- Run `black .`

Start by reading checks.py to follow the existing check patterns, then inspect the relevant files under tests/. Implement the proposed can_redact API and add unit tests for its behavior; run pytest tests/, mypy, and black . to confirm the work is complete.

Written by the indexing model from the issue text.