Add a feature flag for the procedural hooks bc layer

Once core is converted, it will be possible for a site to run without any procedural hook implementations at all.

This requires that none of the installed contributed modules implement procedural hooks. Right?

@chi yes that's right, possible not likely ;) I don't think any sites would be able to actually disable procedural hooks until 11.3 at the earliest or even later, but the end of the 11.x cycle maybe. Also if we want to leave the bc layer in until Drupal 13, we could keep it in 12.x this way, but with a status report mention that it's possible to disable (maybe even with some detection of whether it's being relied upon).

++ to option to skip procedural hooks parsing