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
`os.IsNotExist()` is from before `errors.Is()` existed. The latter is the
recommended approach, as it also recognised wrapped errors.
No functional changes, except for recognising more cases of "does not
exist" errors as such.
Introduce `ParameterMissingError` and `ParameterInvalidError` structs, to
be returned from command executors. These replace free-form `fmt.Errorf()`
style errors.
