run onMount in connectedCallback for custom elements

[hontas]

relates to #1152 and #4522
resolves #2227

When compiling to custom element the onMount hook get called before props have initialized which causes confusion.

Here's how to access props in onMount now:

import { onMount, tick } from 'svelte';

onMount(() => {
  tick().then(() => {
    // props are available
  });
});

This PR changes this to be simpler and map better to connected callback.

import { onMount } from 'svelte';

onMount(() => {
    // props are available
});

[hontas]

Is there a way to re-trigger the checks w/o pushing? Seem to be an error not related to the PR itself.

[nolanlawson]

+1 to this. Right now I'm using the tick() workaround, but it's awkward because my code has to handle the non-initialized state as well as the initialized state. (I.e. have to check that lots of props are non-null, when really they should never be null.)

[Truffula]

#5139 should fix the onMount issue as well as making initial props available on initialization (before onMount) — i.e. #2227. I don't think it affects the onDestroy issues.

[hontas]

Fix moved to #4522