Add typings for module and submodules

[stramel]

Adds typings for the main module and submodules.

[stramel]

The DX of these hooks with typings feels pretty rough. Open to suggestions

[addyosmani]

Thank you for working on a PR adding typings, @stramel. I see there's already been a request for this in #27. I would like to keep this open to give (1) more users a chance to signal if typings are important to them and (2) more reviewers a chance to chime in on DX.

[stramel]

I think to fix the DX of using types, we always return unsupported as a boolean instead of conditionally returning the value or unsupported. useSaveData already functions similarly to this.

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[stramel]

Rebased and updated to match the hook definitions

[anton-karlovskiy]

@stramel cc @addyosmani

Once we've landed #44, I think we can proceed with this PR.

@stramel
Could you please check my comments?

[anton-karlovskiy]

@stramel

It looks duplicated with line 12.
Can we put the type definition file (index.d.ts) inside typings directory, for example, "typings": "typings/index.d.ts"?

[anton-karlovskiy]

@stramel

Could we add semicolon at the end of lines for the purpose of consistency?

[anton-karlovskiy]

@stramel cc @addyosmani

According to typescript handbook, they recommend publishing to the @types organization on npm rather than bundling with the npm package if the package is not written in Typescript.

What are your thoughts on it? :)

[addyosmani]

Thank you for taking the time to work on typings support for the module and submodules, @stramel. I also appreciate the review here, @anton-karlovskiy.

My personal take is that given the package is not written in TypeScript and we have no short-mid term plans to rewrite it in TS for now, it would make more sense for typings to be published in the @types organization rather than bundling it with this package.

@stramel I think this is a reasonable compromise as it unlocks typings for users that want it. Wdyt?

[stramel]

@addyosmani I can do that

[addyosmani]

Cool. Thank you! With this in mind, I'm going to go ahead and close this PR. Let's follow up with a PR adding a link/note in the README to the typings package once it lands in the types organization? :)

[stramel]

Added a draft PR for the typings. I need to clean up the typings and add tests.

Is there anyone who would want to help maintain these types?

@addyosmani @anton-karlovskiy