Refactor: Upgrade Svelte to version 5

[kmcgurty]

Svelte 5 released about a month ago. Probably the biggest changes for this project are:

• Reactive variables must be declared with let myVar = $state(value) instead of Svelte handling everything behind the scenes. The syntax $: is now deprecated. Instead, you use $derived or $effect combined with $state
• The way you create props for components isn't the same (similarly, anything that uses <slot> is deprecated)

You can read the full migration guide here. I don't expect it to be very difficult, and it should work out of the box with Svelte 4 syntax. I haven't tried migrating anything - I can get to it at some point, if no one else wants to do it. It'd also be wise to avoid adding any big app features without upgrading to 5 first.

[Adamcake]

A new version should probably be out soon. Did you want to do this before then?

[kmcgurty]

It's ok to wait. The upgrade won't be too hard since the project isn't that big. I think I'm going to have some time coming up over the next few weeks, I think I can get to this.

[Adamcake]

Done in #172