Follow-on to P2314R4: properly define encoding restrictions

[jensmaurer]

After P2314R4 Character sets and encodings was merged:

https://wiki.edg.com/bin/view/Wg21telecons2021/PaperReviewIssuesTeleconference2021-09-21

Define a term for the encoding restrictions in lex.charset and use that term in the library section to restrict execution character sets / encodings.

[AlisdairM]

Note that those restrictions apply to literal encodings as well as the library encodings.

It would be useful to introduce a named term, apply it to literal encodings in [lex.charset], and move the similar restriction on execution character sets to [character.seq.general].