AppRail improvements

[unlocomqx]

Before submitting the PR:

• Does your PR reference an issue? Discussion
• Did you update and run tests before submission using npm run test?
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• Did you update documentation related to your new feature or changes?

What does your PR address?

This PR improves DX related to using AppRail and AppRailTile.

• You can omit tag="a" when you pass a href to AppRailTile.
• href will be used as the value when no value is provided, resulting in less code repetition and simpler state management.
• Add support for passing a readable to AppRail. Thus making it possible to pass a derived store (to pass in current pathname for example).
• Fix a minor issue where an error is thrown after clicking on an AppRailTile without a store
  cannot read properties of undefined (reading 'set')

Code sample

<script>
  import { AppRail, AppRailTile } from "@skeletonlabs/skeleton"
  import { derived } from "svelte/store"
  import { page } from "$app/stores"

  const selected = derived(page, $page => $page.url.pathname)
</script>

<AppRail {selected}>
  <-- href is used as the value. tag prop is omitted !-->
  <AppRailTile href="/about" label="About"></AppRailTile>
  <AppRailTile href="/admin" label="Admin"></AppRailTile>
  <AppRailTile href="/login" label="Login"></AppRailTile>
</AppRail>

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
skeleton-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 4, 2023 5:50pm

[endigo9740]

@unlocomqx just a heads up I did see this come through, but I'll have to return to this later this week when I have time for a proper review. Thanks for your submission. This looks great with a quick scan!

[unlocomqx]

@endigo9740 It's cool, take your time

Thank you

[endigo9740]

@unlocomqx this seems like a great change, great job. Thanks for your contribution. I'm going to merge this now and this will be part of next week's release!

[unlocomqx]

Great news. Glad to have contributed to such awesome lib

Cheers