Add \protected status to \begin

[josephwright]

As part of #1431, we may to handle \begin tokens at the start of tabular cells. Currently, the underlying \halign will expand such tokens to \protect \begin[space], meaning that any token-by-token code has to either 'reapply' the protect mechanism or handle both \begin and \begin[space]. If \begin is made engine-protected, this is not an issue as the token does not expand here.

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity.

[josephwright]

I'm doing some testing: key question for me is will we want rollback?

[FrankMittelbach]

Not sure, I can't see that it makes a difference -- a real need for a rollback seems unlikely. I think it is probably ok without, but let's hear other opinions.

[davidcarlisle]

I don't think a rollback is needed begin has had a lot of changes making it \protect robust, adding all the hook code etc and has a rollback to a classic 0000/00/00 as well as not robust in 2019/10/01 and no hooks 2020/10/01 so this last change can I think be handled as an in-place update to the last change it seems unlikely there is a lot of existing third party code relying on exactly this version.