Add `leisure=park` to `pois` layer

[mattrobmattrob]

This mimics the leisure=golf_course tag which is present in both the land and pois layers. Park polygon color (possible with the current land layer) helps with relative geospatial association from lower zoom levels and providing names will only increase the value provided by the polygon as zoom level increases.

[pnorman]

Given the other POIs it feels like this was an oversight.

I vote in favour of adding leisure=park to the pois layer.