syntax: nanorc: require whitespace before the start= and end= keywords

Also, drop the unneeded and mistaken 'end=' in the first regex, as
a coloring rule with just end="something" is not valid.  And drop
the requirement for having trailing whitespace or nothing at all,
as it does not constitute a useful restriction.

Also, fold two regexes together, for economy.

syntax/nanorc.nanorc

@@ -20,13 +20,13 @@ color brightgreen "^[[:blank:]]*(syntax[[:blank:]]+[^[:space:]]+|(formatter|lint
 color green "^[[:blank:]]*((un)?(bind|set)|include|syntax|header|magic|comment|formatter|linter|tabgives|extendsyntax)\>"
 
 # Strings
-color brightmagenta "([[:blank:]]|(start|end)=)".+"([[:blank:]]|$)"
+color brightmagenta "[[:blank:]](start=)?".+""
 
 # Control codes
 color bold,pink "[[:cntrl:]]"
 
 # Color commands
-color magenta "^[[:blank:]]*i?color\>" "\<(start|end)="
+color magenta "^[[:blank:]]*i?color\>|[[:blank:]](start=|end=)"
 
 # Comments
 color brightblue "(^|[[:blank:]]+)#.*"