keepURL in pagenotfoundhandler

[hardl]

Description

With this PR, it will be possible to change main url behaviour in case of pagenotfound error, i.e. one can now define to keep the erroneous url in the browsers address bar (e.g. to correct a typo). An example config would look like this:

...
 routing: {
              useHashRouting: true,
              pageNotFoundHandler: (path, anyMatch) => {
                return {
                  redirectTo: '/error',
                  keepURL: true
                };
              }
          },
...

[hardl]

Created as draft, needs test adjustments and docu.