Selector: Detect incorrect escape handling by old Firefox

According to:
https://www.w3.org/TR/css-syntax-3/#ident-token-diagram
the escape doesn't need to be followed by a whitespace only if it's not a hex
digit or a newline. However, old Firefox (3.6 - 5 only) doesn't throw if an
escaped newline is not followed by a space. Fallback to Sizzle in such cases.

Fixes gh-454
Closes gh-456

dist/sizzle.js

@@ -6,7 +6,7 @@
  * Released under the MIT license
  * https://js.foundation/
  *
- * Date: 2019-07-02
+ * Date: 2019-08-20
  */
 ( function( window ) {
 var i,
@@ -846,6 +846,12 @@ setDocument = Sizzle.setDocument = function( node ) {
 				rbuggyQSA.push( ":enabled", ":disabled" );
 			}
 
+			// Support: Firefox <=3.6 - 5 only
+			// Old Firefox doesn't throw on a badly-escaped identifier.
+			el.querySelectorAll( "\\\f" );
+			rbuggyQSA.push( "[\\r\\n\\f]" );
+
+			// Support: Opera 10 - 11 only
 			// Opera 10-11 does not throw on post-comma invalid pseudos
 			el.querySelectorAll( "*,:x" );
 			rbuggyQSA.push( ",.*:" );