Add separate build for custom element

[mskocik]

Would be nice to have it the same way as with Svelecte component. Separate js and css file for fast include without need to build and also to support ES package outside svelte project.

[mskocik]

~Also note in docs, that this polyfill is actualkly needed for webkit~

Irrelevant in v4

[jamess-ais]

I think, If I understand it correctly, this is what I was referencing with the issue I've recently opened

[mskocik]

Reminder for myself: reflect changes in v4 for custom element as well