Prepare Threlte for Svelte 5 upgrade

[micheal-parks]

Overview

This PR gets rid of Threlte imports that have been removed in v8 - the Svelte 5 compatible version.

This is part of a larger slow effort to move us to be ready to upgrade to Svelte 5.

There's actually only one removed import here, and it has to do with event dispatching. Callback props ftw!

[ethanlookpotts]

Love callback props! Thanks for keeping an eye on this! Two thoughts:

1. Do you mind following this PR up with a bump in App just so we don't forget about the changes here and have version drift?
2. Can we think of ways to increase test coverage over these components? There's a lot of critical navigation map infra that is going untested at the moment.

[micheal-parks]

@ethanlookpotts Thank you!

1. For sure, modeled this change so that we could bump immediately 🚀
2. I've been talking to Cousins about this problem lately. The lack of coverage was the result in these components involving maplibre and threlte. @threlte/test doesn't have a canvas + webgl mock, making testing maplibre impossible with the tool, but I think we might have found a potential solution: Vite's new browser mode, which will have a webgl context. I've made some plans in the near future to experiment with adding some browser mode tests specifically here since like you mentioned these components are growing in value and need tests.

[zaporter-work]

LGTM -- don't have much content on the threlte bump though.