Add boundary=protected_area parks

Update the **class** of the **park* layer in a backward-compatible way.

The class for `boundary=protected_area` parks is the lower-case of the
[`protection_title`](http://wiki.openstreetmap.org/wiki/key:protection_title)
value with blanks replaced by `_`.
`national_park` is the class of `protection_title=National Park` and `boundary=national_park`.
`nature_reserve` is the class of `protection_title=Nature Reserve` and `leisure=nature_reserve`.
The class for other [`protection_title`](http://wiki.openstreetmap.org/wiki/key:protection_title)
values is similarly assigned.

Resolve https://github.com/openmaptiles/openmaptiles/issues/760

layers/park/mapping.yaml

@@ -77,6 +77,9 @@ tables:
     - name: boundary
       key: boundary
       type: string
+    - name: protection_title
+      key: protection_title
+      type: string
     - name: area
       type: area
     mapping:
@@ -84,3 +87,4 @@ tables:
       - nature_reserve
       boundary:
       - national_park
+      - protected_area