Make conflicts detectable with errors.Is

[ian-oneleet]

Currently, if there are file-level conflicts that prevent applying a patch, the caller only gets back an anonymous error, so they can't detect whether Apply failed due to a conflict or for some other reason.

This PR changes the error paths for file-level conflicts to return global sentinel errors, and also wraps content-level conflict errors (gitdiff.Conflict) in a sentinel error. With these changes, callers can detect conflicts like so:

		_, err = applier.Apply(ctx, f)
		if err != nil {
			if errors.Is(err, patch2pr.ErrConflict) {
				// there's a conflict
			}
			// something else went wrong
		}

Not too attached to the exact implementation (message wording, error structure, …). Let me know if any of that should change, or feel free to push commits.

Thanks for making this library available!

[bluekeyes]

Thanks, I think it makes sense to expose this information for callers, but I need to think some about the best structure for doing so. I'll let you know if I want to make any adjustments to what you have here.