Add preset for natural=rock

[westnordost]

... which is actually more precisely a free-standing (large) boulder. Already used a whopping 156+k times and documented in the wiki.

Useful for orientation and interesting for climbers.

natural=stone

There is also natural=stone, used 14+k times, also documented in the wiki, but not included in this pull request. It is documented to be a type of boulder, one which is not attached to the underlying bedrock. These often arrived at their current place through glaciation. In English, this type of boulder is called a glacial erratic.

German translation

If you wonder why there are these two tags at all, this is because in German, only boulders attached to the underlying bedrock are called Fels(block), while one that isn't is called Findling. One quote from the German Wikipedia on this topic:

Allgemein wird ein loser Steinblock geowissenschaftlich nicht als Felsen (gewachsen), sondern Stein bezeichnet.

Translation:

In general, a loose stone block is geoscientifically not called a rock (grown), but a stone.

And there you have the origin of natural=rock and natural=stone. A distinction like this is not mentioned in the English Wikipedia articles about boulders, stones and rocks.

It is for this reason why I left out natural=stone aka glacial erratic from this PR, because the distinction between these types of boulders are not part of common knowledge in English.

If you do not see a problem, I can change this to include also the stone.

[bhousel]

Oh neat, I did not know there was a difference.
I'll merge and add both the natural=stone and natural=rock and try to adjust the language a little without making it too jargony or confusing.

[bhousel]

Looks like this now .. It is helpful that the wiki documentation for these is pretty good!