syntax: nanorc: don't color parts of valid strings as if invalid

It's better to color some invalid things as if valid than the other
way around.  So, as strings can validly contain any number of double
quotes, just accept *anything* between the delimiting double quotes
and demand that the closing quote is followed by whitespace or EOL.

syntax/nanorc.nanorc

@@ -16,12 +16,13 @@ icolor brightgreen "^[[:space:]]*extendsyntax[[:space:]]+[[:alpha:]]+[[:space:]]
 icolor brightgreen "^[[:space:]]*(linter|formatter)[[:space:]]+[[:alpha:]]+"
 icolor green "^[[:space:]]*((un)?(bind|set)|include|syntax|header|magic|comment|linter|formatter|extendsyntax)\>"
 
+# Strings
+color brightmagenta "".+"([[:space:]]|$)"
+
 # Colors
 icolor yellow "^[[:space:]]*i?color[[:space:]]*(bright)?(white|black|red|blue|green|yellow|magenta|cyan)?(,(white|black|red|blue|green|yellow|magenta|cyan))?\>"
 icolor magenta "^[[:space:]]*i?color\>" "\<(start|end)="
 
-# Strings
-color brightmagenta ""(\\.|[^"])*""
 # Comments
 color brightblue "(^|[[:space:]]+)#.*$"
 color cyan "^[[:space:]]*##.*$"