Render folders before files in static files picker

[AndyButland]

Prerequisites

• I have added steps to test this contribution in the description below

Fixes: #18698

Description

The linked issue notes that the files are rendered before the folders in the list of child items when using a static file picker. This corrects that, so the folders appear first - which is more expected from viewing file systems and aligns with those coming from the file provider ("wwwroot" folder) that are presented below.

Here's the new ordering:

To Test:

• Add some files and folders into App_Plugins
• From a data type based on a block list, select one of the available blocks and attempt to choose a "Custom stylesheet"