`goto` should error if called with an external route

[Rich-Harris]

Describe the problem

If you do this...

await goto('https://elsewhere.com');

...you get an error:

Cannot use goto with an external URL. Use window.location = "https://elsewhere.com/" instead

But goto('/non-existent-route') doesn't error.

Describe the proposed solution

Make it behave consistently

Alternatives considered

No response

Importance

nice to have

Additional Information

#13303 (comment)

[Rich-Harris]

Might be good to start issuing warnings in the meantime

[rebasecase]

If this is to error for non-app-routes, what do you think about rather than erroring for "external URL" rather as something along the lines of "should be a RouteId"?

For example, say you proxy /admin to a different app before SvelteKit routing. Calling goto('/admin') feels natural, but it will (rightly) error. The term "external" may be confusing here, since this URL isn’t typically thought to be external, it’s just outside of the SvelteKit app itself.