syntax: sql: colorize only single-quoted things as strings

SQL does not know double-quoted strings.  Double quotes can be used
around identifiers, to prevent them being interpreted as keywords.

References:
  https://www.postgresql.org/docs/14/sql-syntax-lexical.html#SQL-SYNTAX-STRINGS
  https://www.postgresql.org/docs/14/sql-syntax-lexical.html#SQL-SYNTAX-IDENTIFIERS

syntax/sql.nanorc

@@ -28,12 +28,16 @@ color brightyellow "\<(CASE|WHEN|IF|THEN|ELSE|ELSE?IF|LOOP|CONTINUE|EXIT)\>"
 color brightyellow "\<(FOR|FOREACH|IN|WHILE|END (CASE|IF|LOOP))\>"
 color magenta "\<(RAISE|EXCEPTION|NOTICE|RETURN)\>"
 
+# Quoted identifiers.  (First scare-color all, then uncolor what is valid.)
+color ,red ""[^"]+""
+color normal ""[[:alpha:]_][[:alnum:]_$]*""
+
+# Strings.
+color green "'([^']|\\')*'"
+
 # Trailing whitespace.
 color ,green "[[:space:]]+$"
 
-# Strings, double-quoted and single-quoted.
-color green ""([^"]|\\")*"|'([^']|\\')*'"
-
 # Comments.
 color red "--.*"
 color brightblue "//.*"