Create a Twig function for route discoverability

Follow-up from #2073811-70: Add a url generator twig extension.

In an effort of compromise, I think we have agreed that a needed helper function (in a separate issue) should be added to determine a list of available routes:
{{ dump(routes()) }} or {{ dump(routes('^user')) }} (for routes that start with user)

This function should allow a RegEx pattern matching argument to filter routes.

This is currently blocked by #2106811: Create route discoverability (back end)