Check: `has_power_level`

Users have to implement manually a check to verify if a user performing a command has the required power level. Since it's a common pattern, we should implement a check for that in `checks.py`.

### Proposed API
```python
def has_power_level(level: int) -> Callable:
async def _has_power_level(ctx: Context) -> bool:
...

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

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

@has_power_level(100)
@bot.command()
async def kick(ctx, user: str) -> None:
```

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

Start with checks.py and the existing check patterns, then add unit tests under tests/ for the proposed has_power_level API. Done means the check enforces the requested power level and mypy, pytest tests/, and black . all pass.

Written by the indexing model from the issue text.