This commit adds a `Loader` type, which can be used to load multiple
source strings. This was done to support the work on modules, but
coincidentally enabled consolidating errors, since now `Config::run`
can take a `&Loader`, and in the event of an error, return and `Error`
that borrows from loaded strings. Multiple error types have been
consolidated, and a bunch of ad-hoc error printing was removed.
When `--init` is passed on the command line, search upward for the
project root, identified by the presence of a VCS directory like `.git`,
falling back to the current directory, and create a default justfile in
that directory.
