Add `team_abbr_mapping` and `player_name_mapping` datasets (#42)

along with code to read datasets

This is a part of #36 

The cleaning functions should go into a separate PR imo

Test with 

```python
import nflreadpy as nfl
nfl.team_abbr_mapping()
nfl.team_abbr_mapping_norelocate()
nfl.player_name_mapping()
```

nflreadpy saves data as parquet, [exported from
nflreadr](https://github.com/nflverse/nflreadr/blob/main/data-raw/data_export_for_nflreadpy.R)