allow <slot> to be part of a slot

[tanhauhau]

Fixes #2079

• allow <slot> to have slot="name".
• currently let: binding on <slot> not supported yet.

[tanhauhau]

Related to #1824, can do

<svelte:component this={Bar}>
      <slot></slot>
      <slot name="header" slot="header"></slot>
</svelte:component>

<script>
	import Bar from './Bar.svelte';
</script>

as a forwarding workaround

[ZerdoX-x]

I want to make some add-ons or wrappers on components e.g
BigButton.svelte

<script>
import Button from './Button.svelte'
</script>

<Button fz="16" h="64" {...$$props}>
  <slot slot="prepend" name="prepend" />
  <slot />
  <slot slot="append" name="append" />
</Button>

But it doesn't work so I have to wrap slots in useless and interfering divs or spans like this:

<Button fz="16" h="64" {...$$props}>
  <span slot="prepend"><slot name="prepend" /></span>
  <slot />
  <span slot="append"><slot name="append" /></span>
</Button>