Add youth center

[westnordost]

Definition

A youth center is a type of community center where juveniles can spend their pasttime after school. See Youth Centers on Wikipedia. So far it has mostly been mapped in Germany and the UK.

Current usage

See Key:community_centre for other possible types of community centres which might be added (later). At the time of writing, community_centre=youth_centre is used 1196 times.
There is a deprecated tag amenity=youth_centre with about 130 usages. If this PR is merged, I'll add another PR for the the upgrade path of that tag.

Possible problem

There is a bit of an overlap with community_centre:for=juvenile/child

https://taghistory.raifer.tech/#***/community_centre:for/juvenile&***/community_centre:for/child&***/community_centre/youth_centre&***/amenity/youth_centre

While these two tags might seem to be in opposition to each other, the wiki doesn't make it look like this is the case. Both wiki sites link to each other and the paragraph for community_centre=youth_centre recommends to

[...] specify the target group with community_centre:for=juvenile or community_centre:for=child. You can also give precise age ranges with min_age=* and max_age=*.

So, the duplication isn't seen as problematic. It's rather that one tag is for the (rough) type of community centre, the other is for specifying more exactly for which people this is a community centre. I just mention this here for later reference.

This PR doesn't add the community_centre:for=juvenile tag because this preset describes a type of community centre, a map feature. community_centre:for on the other hand is a property (field) of a community centre. And in the case of a youth center, it might be for children, or for juveniles, or even defined very exactly by min_age and max_age as mentioned in the wiki.