Implement watching a path

Each inotify instance produces events for paths in its watch list. Each
item in the watch list is identified by its watch descriptor. Different
paths can be watched for different events.

Sources/Inotify/InotifyError.swift

@@ -2,11 +2,14 @@ import CInotify
 
 public enum InotifyError: Error, Sendable, CustomStringConvertible {
 	case initFailed(errno: Int32)
+	case addWatchFailed(path: String, errno: Int32)
 
 	public var description: String {
 		switch self {
 		case .initFailed(let code):
 			"inotify_init1 failed: \(readableErrno(code))"
+		case .addWatchFailed(let path, let code):
+			"inotify_add_watch failed for '\(path)': \(readableErrno(code))"
 		}
 	}