This introduces TypeScript type hints that cover the functions and

tagged template literals processed by this plugin. It also includes a

shim for Underscore that helps merge the definitions for Underscore's

_ object and the _ gettext alias.
Consumers will want to reference the types in individual files or in a

global.d.ts by including the line:

/// <reference types="babel-plugin-django-gettext"/>

Made use of these new definitions in Djblets and Review Board.
Unit tests pass.