(#19201) Add concat function to join two arrays

(#19201) Add concat function to join two arrays

Without this patch applied there is no easy way to append one array to
another.  This is a problem because it is often desirable to join two
arrays without flattening the contents into a single, one dimensional
array.

This patch addresses the problem by adding a `concat()` function which
takes two arguments.  The arguments will be concatenated together and a
new array returned to the caller.

Reviewed-by: Jeff McCune <jeff@puppetlabs.com>