This is an initial start to using TypeScript in our non-core Ghost packages.
- Adds a prepare script to build the project after installing deps
- Adds an initial tsconfig.json which is compatible with our node env
- Migrates all of the code to TypeScript, including tests
- Updates tests to use ts-node so that we don't need to compile the tests
- ts-node is installed at the top level because the env is weird with lerna and
doesn't work otherwise
- Updates the yarn dev script to build the project with the --all and --revisions flag
