Store spaces/tabs as actives in v-type arg

[josephwright]

READ ME FIRST: Please understand that in most cases we will not be able to merge a pull request because there are a lot of internal activities needed when updating the LaTeX2e sources. If you have a code suggestion please discuss it with the team first.

Internal housekeeping

Status of pull request

• Feedback wanted
• Under development
• Ready to merge

Checklist of required changes before merge will be approved

• Test file(s) added
• Version and date string updated in changed source files
• Relevant \changes entries in source included
• Relevant changes.txt updated
• Rollback provided (if necessary)?
• ltnewsX.tex (and/or latexchanges.tex) updated

[josephwright]

PR extended to include revision of catcodes from v-type argument - you don't get 'normalisation', rather 'whatever was not in \dospecials left alone', but that feels consistent with \verb and realistically is probably the best one can do unless you go LuaTeX-only (and can rescan with a clear catcode table).

[josephwright]

@muzimuzhi For context, the change of catcodes should make it easier to get a 'matching' result when #1525 is merged: both are verb-like, so it would be best if they match as far as possible

[FrankMittelbach]

approved with some comments on text (and a query)