PMTilesReader has no is_vector property

The example in the [README.md](https://github.com/developmentseed/async-pmtiles/blob/d019ddd6e34353620333f12fcf8c2a0d9758c7a1/README.md) includes this snippet:
```
# Is the data a Vector Tile Archive
assert src.is_vector
```

The problem is there is no `is_vector` property/attribute on the `PMTilesReader ` so this results in an `AttributeError` being raised instead.
This property seems to intend to check that the file_type property is equal to `TileType.MVT`.

Did you intend to provide the property?

Start with the README.md example and locate the PMTilesReader implementation. Check how its file_type is represented relative to TileType.MVT, then verify that the README assertion works without raising AttributeError; add or update coverage if the project already has a relevant test location.

Written by the indexing model from the issue text.