Support for blocking navigation (useBeforeLeave)

[Brendan-csel]

Adds this...

useBeforeLeave

useBeforeLeave takes a function that will be called prior to leaving a route. The function will be called with:

• from (Location): current location (before change).
• to (string | number}: path passed to navigate.
• options (NavigateOptions}: options passed to navigate.
• preventDefault (void function): call to block the route change.
• defaultPrevented (readonly boolean): true if any previously called leave handlers called preventDefault().
• retry (void function, force?: boolean ): call to retry the same navigation, perhaps after confirming with the user. Pass true to skip running the leave handlers again (ie force navigate without confirming).

Example usage:

useBeforeLeave((e: BeforeLeaveEventArgs) => {
  if (form.isDirty && !e.defaultPrevented) {
    // preventDefault to block immediately and prompt user async
    e.preventDefault();
    setTimeout(() => {
      if (window.confirm("Discard unsaved changes - are you sure?")) {
        // user wants to proceed anyway so retry with force=true
        e.retry(true); 
      }
    }, 100);
  }
});

NOTE LIMITATIONS

This PR only adds support to block navigations originating from the router (eg <Link>, useNavigate()).

Blocking navigations originating in the browser (back/forward, plain anchors) can also be done and requires either a custom path/hash integration, or solid-router to extend its built-in integrations. This branch includes this addtional behavior.

Finally, users may wish to block full page document navigations (ie using window beforeunload event) but that should be handled in application code directly (outside the scope of the router).

[Brendan-csel]

Have added "from" path to event args as it proved useful in real-world testing.

[Brendan-csel]

Hmm, I think there is an issue here requiring a bit of a code restructure. I set up the leave handlers globally - but they should be per <Router>. I'll work on that.

[Brendan-csel]

Okay the latest commit makes the BeforeLeave subscribers specific to the Router instance.

BeforeLeave management can be created by either:

1. the Router - which means it only supports blocking router initiated navigations.
2. the integration - which means browser initiated navigations can also be blocked (back/forward, etc).

Note that 2 still requires a change to the integrations which I haven't included in this PR - but do have in this branch (and are using in production O_O!!)

[ryansolid]

This looks really good, and it taps in before Transitions so I think we are good here. Love it.