The function returns true if and only if the argument is a WKT point.
This can be checked efficiently by checking the datatype bits of the ID.

Note that this function is not part of the GeoSPARQL standard. We add it
because we need it for https://github.com/ad-freiburg/qlever-petrimaps
to fetch alll WKT literals that are not points efficiently.

Reason: the predicate is called `geo:asWKT`, so the new name looks more
consistent with that.

1. It was not a good idea to make this a `geof:` function for two
   reasons: First, it's not a standard `geof:` function. Second, and
   worse, the function does not actually check whether the argument is a
   WKT point, it only checks whether the argument is a WKT point that is
   stored in the `Id`. There are also WKT points that are not stored in
   the `Id` for various reasons (notably, when the coordinates are out
   of range). It is therefore now a QLever-internal function with the
   name `ql:isGeoPoint`, following the name of the `Datatype`.

2. This is our first QLever-internal function, so far we only had
   QLever-internal predicates (like `ql:has-predicate` or `ql:langtag`)
   and entities (like `ql:@en`, which technicall is not a valid IRI, but
   that is not relevant here). This required new constants in
   `Constants.h`, which prompted me to give better names to the other
   constants there related to QLever-internal stuff. This, in turn,
   required renaming in quite a few files. It looks like a lot, but
   almost all of it is really just straightforward renaming.