Add `groupGlobalsBy()` helper function

[zanerock]

In order to support a grouping globals by category, let's add a groupGlobalsBy(groupByFields, options) function. Currently, you can only really group by category if the grouped identifiers are part of a module (because the groupBy() works explicitly with child identifiers so there must be a parent involved). Adding groupGlobalsBy() will allow global identifiers to be grouped by category as well.