On the use of two-dot range notation

The two-dot range notation (e.g. 10..35) is used in [charconv.to.chars] p5. Since we don't have such a syntax, it would be more readable to use a dash "–" 😄 (e.g. 10–35).

This notation is also used in [tab:lex.charset.basic]. (The ".." notation is widely used in the Unicode Standard so it might be OK to use the notation here, though IMHO regular spaces around ".." look too wide.)