apiVersion vs branch (v1.2 vs v1.3)

The [`app.apiVersion`](https://github.com/aseprite/api/blob/main/api/app.md#appapiversion) is not a good way to check for API features when we have this branch of versions (e.g. v1.2 and v1.3). We think that we could add a new object to check features:

```lua
if app.features.hasTilemapLayers then
...
end
```

In case of a property that doesn't exist is used (e.g. `app.features.nonExistentFeature`) it just return false (instead of a Lua error).