Adding ability to use a 404 route

[chrisgubbels]

In page.js you can define a 404 route, using '*' as your path.

In the router, this route's callback is still called after the callback of any route that might also have a triggered routePath.

What we want is for the * path to only be activated if the context has not been handled.

To test:

1. Tests should pass.
2. Checkout this branch in digital-tools, yarn link web-component-router and run yarn serve in the example-dashboard-app
   2a. Go to the dashboard page, the 404 should not show. Add /sdfahsdkjfh or something to the url, the 404 page should show.

[barronhagerman]

I agree these changes would be necessary for a "*" fallback route, in order to avoid always showing that route. However, I think it could only be used if there are no other applications in the same domain.

For example, consider an application on https://localhost/app1 with a "*" route to show a 404 error page. If you click on a link to https://localhost/app2 in the app1 app, I think the 404 page would be displayed rather than redirecting to the expected location because the 404 route handler would set context.handled = true, and this line in the unhandled handler would never be executed:

web-component-router/lib/page.js

Line 618 in 02ab3e0

window.location.href = url.toString();