Provide a means to prevent files/directories/patterns from being tracked as part of a directory artifact

[kevin-hanselman]

The main benefit is that directory artifacts could be created from directories whose contents should not be entirely tracked by Dud.

Option 1: Support .dudignore files that work similarly to .gitignore files. This is likely most useful when there's a pattern that shouldn't be tracked project-wide.

Option 2: Replace Artifact.DisableRecursion with a .gitignore-style list defined in the stage YAML itself. In this approach, YAML files remain standalone; no separate .dudignore file affects the definition of the stage.

Decoupling ignored patterns from the stages/artifacts themselves has strong pros and strong cons.