upgrade svelte3 to svelte4 and migrated to the latest sveltekit with svelte package

[JinIgarashi]

@GeoffCox Thanks for developing this awesome svelte library!

There is a problem of using this library in my repo in the latest sveltekit environment. I migrated the package to svelte4 and sveltekit.

I did some minor changes on your code for fixing lint error, but I did not change anything about the current interface.

I did following:

• migrated to sveltekit by merging demo/package folder
  • I created by npm create svelte@latest ., then simply migrated all files. demo folter is moved to under routes, package folder is moved to under lib.
• svelte 3 was migrated to svelte 4. I tested in my local, the code works good without changing anything for svelte 4.
• due to this migration, rollup was removed, and svelte package library, which is the official and standard way to develop a svelte package, is used instead.

I deployed my version to @watergis/svelte-splitter on npm, and this works fine in my project.

If everything is good, could you merge this and bump a major version for @geoffcox/svelte-splitter?