Add declaration files to support typechecking

[sync-by-unito]

When installing and importing @numbersprotocol/nit 1.6.0 from npm in a Typescript project, there will be a warning "Could not find a declaration file for @numbersprotocol/nit", and type is not supported. .d.ts declaration files should be added to the released version to support typechecking in Typescript.

Onc

┆Issue is synchronized with this Asana task by Unito
┆Created By: James Chien