Possibly consolidate machine name generation API

Problem/Motivation

Follow up from 🐛 Block name collision on theme creation Fixed

Comment #75 from @alexpott

Having a swappable block machine name generator service feels interesting. I'm not sure about this being part of the API. Also having {@inheritdoc} as a class doc feels wrong. Note sure I've seen that before.

Plus I wonder how this fits in with
#1643386: Strip useless "_" at beginning and end of JS-transliterated machine names which is pointing towards a need to consolidate the various places we generate machine names in core

#87 from @xjm

Regarding #75.1 and whether we should consolidate machine name generation APIs, I think that's out of scope for this issue, since this is still fixing a major-possibly-critical data integrity bug. First of all, we'll need separate implementations in JS and PHP regardless -- the JS generates a suggested machine name for the user, but the PHP has to do its own validation either way.

Steps to reproduce

TBD

Proposed resolution

TBD

Remaining tasks

TBD

User interface changes

TBD

API changes

TBD

Data model changes

TBD

Release notes snippet

TBD