Add TypeScript definitions for store (fixes #1207)

[PaulBGD]

I'm not sure how to handle this for both the store.js and store.umd.js. I think in most cases TypeScript users will use store.js anyways, so I don't think it's a big issue.

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@@            Coverage Diff            @@
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  Files             ?      127           
  Lines             ?     4558           
  Branches          ?     1503           
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  Hits              ?     4177           
  Misses            ?      159           
  Partials          ?      222

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[PaulBGD]

Remind me to not write definitions at night, I'll look at your suggestions in the morning.

[Kiho]

I could not find official type definitions for svelte anywhere, can anybody give me a reference?

[stalkerg]

@PaulBGD any update?

[Rich-Harris]

finally got round to updating this