"Hidden" standard with helper tools for WPCS maintainers

There are a number of lists in WPCS which should be updated on a regular basis - on every WP release - and gathering the information needed for this manually is a big maintenance burden.

Think:

• List of WP native global variables - also see #924
• List of deprecated hooks
• List of deprecated functions
• List of deprecated function arguments
• List of all class names in the correct case
• ... etc

It would be relatively easy to create a few sniffs which could be run over WP Core to gather this information.
Those sniffs however would have no value for the end-users of WPCS, only for WPCS developers.

So, what I'm proposing is as follows:

• Add a new folder AnalyseWPCore with a ruleset.xml and sniff directory.
• Add this folder to .gitattributes to be export-ignored. This way it will not be available in released versions of WPCS, but will only be available in git clones.
• Over time add some custom sniffs to that directory which can gather the information we need to easily update the above mentioned lists.

Potentially future enhancements:

• Add some custom reports which would pre-format the output of certain sniffs in a way that the output could just be copy/pasted into the right lists.
• Add a bash script to run the reports in one go and potentially even update the lists in WPCS automatically.
• Potentially run the reports via GH Actions on a cron schedule an automatically open an issue/PR/post to Slack when an update is needed.
  Note: we need to be careful about running against WP trunk though as we don't want WPCS to start flagging things which haven't been released et.

Opinions ?

FYI: I've already got a variation of the PrefixAllGlobals sniff ready for instance to allows us to update the global variables list easily.