Add state() method handling for components

[emilos]

Hey,

This PR is a proposal of API extension that would allow setting the Store inside of the top level component (previously discussed briefly in gitter).

import { Store } from 'svelte/store'
export default {
  store () {
    return new Store({ foo: 'bar' })
  }
}

The motivation for such change is making it easier to consume the component in other apps.

Instead of:

import Component from '@foo/bar'
import { Store } from 'svelte/store'

const store = new Store({ foo: 'bar' })
new Component({
  target: document.getElementById('foo'),
  store
})

it would become:

import Component from '@foo/bar'

new Component({
  target: document.getElementById('foo')
})

where the store would become an implementation detail instead.

Please let me know if there's anything missing (e.g. specs of some type) and I'll add :).

Thanks!

[codecov-io]

Codecov Report

Merging #1028 into master will decrease coverage by 0.01%.
The diff coverage is 90%.

@@            Coverage Diff            @@
##           master   #1028      +/-   ##
=========================================
- Coverage   92.01%     92%   -0.02%     
=========================================
  Files         119     119              
  Lines        4356    4363       +7     
  Branches     1402    1406       +4     
=========================================
+ Hits         4008    4014       +6     
  Misses        148     148              
- Partials      200     201       +1

Impacted Files	Coverage Δ
src/validate/js/propValidators/index.ts	100% <ø> (ø)	⬆️
src/generators/Generator.ts	94.51% <100%> (+0.02%)	⬆️
src/generators/dom/index.ts	95.42% <100%> (+0.02%)	⬆️
src/generators/server-side-rendering/index.ts	95.83% <83.33%> (-1.23%)	⬇️

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[Rich-Harris]

Awesome, thanks so much!