CLI tool to rename file or tree (github#25306)

* CLI tool to rename file or tree

* it works

* progress

* progress

* wip

* wip

* wip

* debugging

* fixed

* remove console logging

* add support for also updating learning-tracks

* cope with the 'children' key in the root content/index.md unicorn

* update childGroups too

* guardrails

* refactor

* tidying up arguments

* refactored for single files

* nicer error in pre-validation of the first file argument

* regression fix

* fix for moving directories

* delete lib/redirects/.redirects-cache.json if present

Author: peterbe