Add >=jest@28 as a peerDep (#152)

* Add >=jest@28 as a peerDep

This commit adds a peerDep of >=jest@28.

jest@28 includes jestjs/jest#11961, which
supports `exports` in `package.json`.

Without this, jest users that use jest@27 or below will throw an error
when firebase-functions-test attempts to import several named exports.

Author: TheIronDev