Don't allow creating tiles with negative zoom levels.

Nobody has claimed this yet.

This was first brought up in https://github.com/mapbox/mercantile/issues/122#issuecomment-788116884 which mentions the use case of calculating the parent of a zoom 0 tile:
```python
>>> mercantile.parent(mercantile.Tile(0,0,0))
Tile(x=0, y=0, z=-1)
```

The zoom 0 parent tile case was fixed in https://github.com/mapbox/mercantile/pull/130, the changelog entry from that PR says:
> The Tile constructor in mercantile 2.0 will refuse to make tiles with X and Y
indexes outside of the range 0 <= value <= 2 ** zoom. It will also require
indexes to be integers.

But this constraint actually isn't true, here is a really easy way to break it:
```python
import mercantile

zoom = -1
min_index = 0
max_index = 2 ** zoom

tile = mercantile.Tile(x=10, y=10, z=zoom)
assert min_index <= tile.x <= max_index # raises AssertionError
assert min_index <= tile.y <= max_index # raises AssertionError
```

The easiest way to make the aforementioned constraint actually work is to prevent users from creating tiles with a negative zoom level, and in my opinion this is really what the constraint is trying to do. Mercantile is a library for `Spherical mercator coordinate and tile utilities`. By definition the lowest zoom level in the mercator grid is zoom 0, at which point a single tile covers the the entire world. So even allowing users to provide a negative zoom seems to break the original intention of the library, and can lead to some very unintuitive behavior that doesn't align with how the mercator grid works in reality:

```python
tile = mercantile.Tile(x=0, y=0, z=-1)
children = mercantile.children(tile)

# Mercator grid by definition only has a single Z0 tile
assert len(children) == 4
```

Start with the Tile constructor and the parent/children behavior described in the issue. Add coverage for rejecting a negative zoom level and verify that zoom 0 remains the lowest valid level, including the reported parent and children cases.

Written by the indexing model from the issue text.