Create a way to query the number of co-locales on a node

[bradcray]

In a discussion in https://github.com/Cray/chapel-private/issues/6844, we discussed that it could be useful to have some sort of query available to Chapel programmers, like here.numCoLocales() to query how many co-locales are running on a given node.

writeln(here.numColocales(), " locales are running on this compute node");

In that context, it came up because you could imagine wanting a benchmark or application to report its speeds as a rate per node vs. locale and may want to reason about the number of co-locales in doing so.  I could imagine it coming up in other cases as well, such as algorithms that may want to be sensitive to co-location of co-locales.

For starters, I'm imagining only supporting this on top-level locales, not sub-locales like GPUs.