Add leisure=picnic_table to a future version of the schema

[SomeoneElseOSM]

Currently at https://taginfo.openstreetmap.org/keys/leisure#values there are 434k of these worldwide, so it's a frequently used value. Usage isn't spread evenly (it's most common in Europe, North America and Australasia) but it is used across the world https://taginfo.openstreetmap.org/tags/leisure=picnic_table#map . It's documented at https://wiki.openstreetmap.org/wiki/Tag%3Aleisure%3Dpicnic_table .

It would add value to a general purpose map style that may want to use the Shortbread schema.

It would make sense to add it to: https://shortbread-tiles.org/schema/1.1/#layer-pois

to mirror the way that amenity=bench is handled.