Follow-on to P2314R4: properly define encoding restrictions

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.