syntax: rust: do not colorize as string the text between two strings

Strings may not contain a double quote unless it is escaped.

Reference:
  https://doc.rust-lang.org/reference/tokens.html#string-literals

(This will still not colorize multi-line strings with an unescaped
newline, will still colorize "this\" as if it were a valid string,
and will still miscolor things when there is another string after
the closing quote of a two-line string.  But those things can't
be helped -- line-based regexes cannot emulate a full parser.)

This fixes https://savannah.gnu.org/bugs/?61361.
Reported-by: Elias Jonsson <e@ejon.eu>

Bug existed since version 2.6.1, since the Rust syntax was introduced.

syntax/rust.nanorc

@@ -22,8 +22,8 @@ color magenta "[A-Z][A-Z_0-9]+"
 color magenta "[A-Z][A-Za-z0-9]+"
 
 # Strings
-color green "".*""
-color green start="".*\\$" end=".*""
+color green ""([^"]|\\")*""
+color green start=""([^"]|\\")*\\$" end=".*""
 ## NOTE: This isn't accurate, but matching "#{0,} for the end of the string is too liberal.
 color green start="r#+"" end=""#+"