feat(typing): adds more typescript definitions

[sbelzile-nexapp]

The types I added to my project, if that can help you or somebody else :-)

Might be wrong on a few points (I don't use all of them so: to be reviewed).

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[TheComputerM]

Hey, Thank you for your contribution. There are a few things that can be improved. I suspect that sveltejs/rfcs#37 will be approved so instead of directly importing SvelteComponent from svelte it is better to create a shared.d.ts that exports a class similar to the one in the RFC to make the transition smoother once the rfc is approved.

You can take a look at:

• sample shared.d.ts
• sample component.d.ts

[sbelzile-nexapp]

Did not know about this RFC: it's a good thing :-P. Anyway, I updated the code!

[TheComputerM]

Thank you very much, will merge this soon.