Initial implementation with some basic tests.
The API command only accept one source and destination path, which must
be absolute. If the destination already exists, it will not be
ovewritten, and an error is generated.
JS job definition code using this new command can easily handle list of
paths and/or relative paths if needed.
Reviewed-on: https://projects.blender.org/studio/flamenco/pulls/104220
Introduce `ParameterMissingError` and `ParameterInvalidError` structs, to
be returned from command executors. These replace free-form `fmt.Errorf()`
style errors.
