Accept str values for config, target_dir in file tool

[maddenp-noaa]

Synopsis

1. In addition to the copy() and link() convenience functions we provide via uwtools.api.file, we now also provide direct access to the Copier and Linker objects, so that external drivers can instantiate them and use their tasks directly, e.g. yielding them as dependency tasks in their own iotaa tasks. We already ensured that str values provided by users in copy() or link() calls are converted to the Path objects expected by Copier and Linker. But it will be convenient for users to be able to provide str values when they directly call the constructors of those classes. So, the necessary conversions are added.
2. We previously renamed FileCopier -> Copier (and similarly FileLinker -> Linker) to avoid the awkward repetition in e.g. uwtools.file.FileLinker. FileStager -> Stager is added here.

Type

• Code maintenance (refactoring, etc. without behavior change)

Impact

• This is a non-breaking change (existing functionality continues to work as expected)

Checklist

• I have added myself and any co-authors to the PR's Assignees list.
• I have reviewed the documentation and have made any updates necessitated by this change.

[christinaholtNOAA]

LGTM.