The directory created by `T.TempDir` is automatically removed when the
test and all its subtests complete.
Reference: https://pkg.go.dev/testing#T.TempDir
Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>
weldr's store is quite complex and handled serialization itself. Move
that part out into a separate package `jsondb`.
This package is more generic than the store needs: it can write an
arbitrary amount of JSON documents while the store only needs one:
state.json. This is in preparation for future work, which introduces a
queue package that builds on top of `jsondb`.
