Add typescript tests with example usage as sanity check and hook into testscript

[rosskevin]

• Add typescript usage tests for sanity and hook into ci to assist in keeping up to date.
• Fix several errors in the type definition

Closes #654

[rosskevin]

Note, this test proves and fixes a few errors I am seeing downstream in my app.

ts users are likely broken due to the bad context type import until this is merged and released.

[coveralls]

Coverage remained the same at 75.157% when pulling 5bcf60f on rosskevin:ts-tests into 0bf4437 on i18next:master.

[rosskevin]

I'm still threading through my codebase and finding some fixes, let's hold this until I get a clean run locally. I'll update this issue when ready.

[rosskevin]

This PR is ready to merge.

[rosskevin]

@jamuhl if we can get this merged and released, I would appreciate it.

Incorrect types included in-repo are not easily ignored, whereas they are if they are from definitelytyped (just don't include them). This PR fixes a breaking change for ts users and should get us back to working.

[jamuhl]

published in react-i18next@9.0.1

[VincentLanglet]

@rosskevin Do you have time to test also the file for hook type ?

You broke them with your PR, cf : #658

[rosskevin]

@VincentLanglet I don't use the hooks yet, but feel free to add usage tests similar to how I did the index.