This significantly speeds up `yarn lint` for me. It also ensures we’re
listing all source files in the project, including the `.eslintrc` files
and any other files that may be present.

I've updated the return values in `sync/index` to sensible values based
on how they're being called.

I've updated the type of `msg.value` to be `string | number | null` to
match the values expected by `serializeValue`

actual-server does not need to import the full actual-app/api package.
It can import only the CRDT stuff.. so I'm extracting it into a new
package to reduce the size of actual-server and make the link between
things more transparent.