Make getFixedT type-safe

[pedrodurek]

Closes #1856

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 remained the same at 91.157% when pulling f46311d on pedrodurek:type-getFixedT into 96613fa on i18next:master.

[adrai]

Do you want me to release a new patch version or should we wait?

[pedrodurek]

@adrai I'll fix this one first, so we merge together

[adrai]

included in v22.0.3