Typescript - Support context

[pedrodurek]

Closes i18next/react-i18next#1617, i18next/react-i18next#1333, i18next/react-i18next#1541

Limitations:

1. Context and Plurals: Currently, due to a TypeScript limitation, it's required to pass a count while using context with plurals. I still need to figure out an workaround for it.
2. Filtering Keys Based on Context: At present, to filter appropriate keys according to the context, it is necessary to assert the context using as const to infer the proper string literal value rather than string. This'll be addressed here.

Checklist

• only relevant code is changed (make a diff before you submit the PR)
• run tests npm run test
• tests are included
• commit message and code follows the Developer's Certification of Origin

Checklist (for documentation change)

• only relevant documentation part is changed (make a diff before you submit the PR)
• motivation/reason is provided
• commit message and code follows the Developer's Certification of Origin

[coveralls]

coverage: 92.36%. remained the same when pulling f8b4036 on pedrodurek:typescript/support-context into 549ada8 on i18next:master.

[adrai]

lgtm, but you're the expert ;-)
Should we wait for this to merge?

[pedrodurek]

Yes! Let's wait, I should put a PR up for it later today

[adrai]

this PR is obsolete now, right?

[adrai]

already included in #2007 and released with v23.4.0