Add `mget(img, x, y)` function to get map sprite at coordinates
**Is your feature request related to a problem? Please describe.**
Easy way to know what sprite is at certain map coordinates, for collisions and other needs.

**Describe the solution you'd like**
Basically expose the `get_tilemap` function from 10_platformer example in the pyxel namespace.

The name `mget` comes from pico8 (meaning _map (tile) get_), it could be `tget` if you prefer _tile get_ or `tmget` or something else.  I only care that function names are internally consistent 🙂 (which makes me suggest that the `pget` method in tilemap is misnamed!)

The doc will have to make clear that `img, x, y` are tilemap parameter, not screen coordinates — this tripped me up for some time when I started pico8.  But everyone learns that, or it’s not a problem if you blit the map at 0,0 and use `camera` for scrolling.

**Describe alternatives you've considered**
Copying the `get_tilemap` function forever in all my projects 🙂 

**Additional context**
Would also be great to have `mset(img, x, y, tile)` for changing the tilemap in memory during the game.

(Then the next step is to add sprite flags!  These are a neat way to give more power to artists doing sprites and maps, without having the developer hard-code lists of tiles in collision etc. code.)