refs https://github.com/TryGhost/DevOps/issues/45
- this switches the monorepo over to using Nx instead of Lerna, because
we don't currently need the versioning+publishing capabilities
- this also adds an `nx.json`, which allows us to enable task caching
- also adds `build:ts` to the TS projects, which is cached for fast execution
- how these interact with the dev.js script will hopefully soon be
reworked to be a better experience
refs https://github.com/TryGhost/DevOps/issues/45
- this moves the bulk of the tsconfig file to the `ghost/` folder and
simply extends this file in each of the TS lib packages we currently
have
- this makes it a lot easier to make single changes to our TS config
