"onBeforeNavigate" navigation interceptor and "onNavigate" lifecycle function

[pavelbelyaev98]

This pr closes issues #2974 and #560

This pr introduces a onBeforeNavigate interceptor and a onNavigate lifecycle function.

onBeforeNavigate

This is a navigation interceptor that can be used to either look up the incoming URL we are going to navigate to or even prevent the navigation from completing by cancelling the navigation request. It works only for internal app navigation.

The navigation interceptor is called onBeforeNavigate and is being called each time we use the following methods:

• goto()
• popstate events
• internal anchor tags clicks

The interceptor can be used in the following way:

<script>
  import { onBeforeNavigate } from '$app/navigation';

  onBeforeNavigate(async (url) => {
	// the URL we are going to navigate to
	console.log(url);

	// cancel or allow navigation in the return statement
	return false;
   });
	
</script>

It can be used with and without async, also It works without a return.

onNavigate

Additionally, the pr adds a onNavigate lifecycle function that runs when the page mounts, and also whenever SvelteKit navigates to a new URL but stays on this component.

<script>
  import { onNavigate } from '$app/navigation';

       onNavigate(() => {
	  console.log("Navigation compelted!");
        });
	
</script>

---

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Tests

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🦋 Changeset detected

Latest commit: eea4279

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Minor

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[pavelbelyaev98]

@benmccann I've implemented the required changes, this can be reviewed again.

[bluwy]

Note: I've put this into next maintainer's meeting list to discuss the path forward of it.

[netlify]

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[pavelbelyaev98]

Closing this in favor of #3293