Add S3 methods for sorting & filtering hierarchical tables

[edelarua]

What changes are proposed in this pull request?

• Added S3 methods tbl_sort.tbl_hierarchical() and tbl_filter.tbl_hierarchical() to sort and filter hierarchical tables.

Closes #2096

To consider:

• Do we need the .stat argument or just require users to include "n" in their table if they want to use it?
• When filtering, if there are no rows left in a section do we still keep the summary row for that section if it it meets the criteria?

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[ddsjoberg]

This is awesome @edelarua ! Thank you!!

When we're back from the holidays, let's chat about the user API. Maybe we can make it easier to implement from their end with respect to specifying the expression for filtering, for example. For example, would it be easier for them to specify the filter like p >= 0.05 instead of t = 0.05, .stat = "p"?