In UTF-8 locales, the ranges A-F and a-f include several accented
characters beyond "ABCDEF" and "abcdef", such as the Croat č and ć.
This fixes https://savannah.gnu.org/bugs/?61487.
Bug existed since the beginning of each of the syntaxes,
for the C syntax since version 1.3.6, commit 159bdfdc.
Also, trim some whitespace and group one rule better.
Also, fix a stray closing parenthesis in the JSON syntax,
and add the missing slash to the possible escaped characters.
(Reference: https://www.json.org/json-en.html.)
Also, improve the ending of multiline strings in the Rust syntax.
In many places a carriage return is not valid whitespace and should
thus not be colored as such. In some of these places a vertical tab
or form feed is maybe valid whitespace, but it would be ugly or even
wrong to color them because they are not part of the subsequent
comment or keyword.
This fixes https://savannah.gnu.org/bugs/?60456.
Benno Schulenberg