$$slots to check whether slots is provided

[tanhauhau]

Related #2106

Introducing $$slots

you can use $$slots to determine whether a slot is provided. $$slots is an object, where the keys are the only slot names that are provided and value is true.

for example, if you have a component, Modal,

<Modal>
   <div slot="a">hello</div>
</Modal>

will give you $$slots = { a: true }, while

<Modal>
   <div slot="a">hello</div>
   <div slot="b">world</div>

will give you $$slots = { a: true, b: true }.

You can use it to conditionally render a slot

{#if $$slots.b}
    <div>
         <slot name="b" />
    </div>
{/if}

- can't get the slotted elements at this point, because they are created after instance function.
- A viable alternative is to provide a callback to get the slotted elements after mount.
- the syntax <slot bind:this={slottedElements} /> suggested in #2106 (comment) becomes more natural that way

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Tests

• Run the tests tests with npm test or yarn test)

[Conduitry]

Hi @tanhauhau sorry this has been sitting so long, even after we decided we should go forward with this. Would you mind resolving the conflicts with master please? I'm also not sure I understand the bullet points in the PR description. Are there limitations that should be kept in mind when using this?

[tanhauhau]

@Conduitry i've resolved the conflicts. thank you for looking into this.

also, can help look at #4556 too? :)

[tvanc]

When can we expect to see this in a release? Currently working on a problem where this would be ideal