Add a new function, JSON_PRETTY, which transforms a JSON document into
a more human-readable form with newlines and indentation.

The following changes are backported from trunk:

WL#9191: Add JSON_PRETTY function
Bug#22284168: REMOVE NOW REDUNDANT SPACE RESERVATION IN JSON_DOM.CC
Bug#24586888: SELECT LENGTH(JSON)

Change-Id: I3a79f987e7c4a0a8c94fb685273909669ae3310c