Add type for Formatter.addCached

[tnortman]

Adds TS type for Formatter.addCached

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 3425046 on tnortman:master into f00b57c on i18next:master.

[adrai]

Thank you for your contribution.
v22.0.4 has just been released.