Add boundary=protected_area to park layer

[zstadler]

Currently, boundary=protected_area areas are not included in the park layer.

The OSM wiki pages for boundary=national_park and leisure=nature_reserve say under "Similar tags":

boundary=protected_area is a more recently introduced tag with a more verbose tagging scheme which can deal with all kind of protections and protection levels. It attempts to encompass both nature reserves and national parks, and is used more and more.

Back in 2016, @lukasmartinelli said:

In the park layer we would then have leisure=natural_reserve, boundary=national_park and boundary=protected_area.

I have enhanced the park layer to include the missing areas, as this is the OSM mapping standard in my country.

Here is the new description of the class field where an attempt was made to unify the different tagging options:

Use the class to differentiate between different parks.
The class for boundary=protected_area parks is the lower-case of the 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 values is similarly assigned.

I will be happy to share this enhancement using a PR for this issue.

[TomPohys]

Hello @zstadler, your PR is very welcome. Can be part of v3.12 release.

Connection to #773 and #776