Make Ditto a monorepo

This is a first pass at a complete restructuring, which for now just moves Ditto's source files from src/ to packages/ditto/.

As long as I did not mess up any paths, then this should work fine.

The main package is called @ditto/ditto. Later I will start breaking Ditto up into separate packages, like @ditto/core, @ditto/mastoapi, etc.

Related: #291