[Enhancement] Optional+Nil extension (#17)

This PR contains the work done to add some properties to the `Optional` primitives. In addition, some work has been done to improve some existing codebase as well.

Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Reviewed-on: #17

Sources/Core/Extensions/Optional+Nil.swift

@@ -0,0 +1,23 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftLibs open source project
+//
+// Copyright (c) 2023 Röck+Cöde VoF. and the SwiftLibs project authors
+// Licensed under the EUPL 1.2 or later.
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftLibs project authors
+//
+//===----------------------------------------------------------------------===//
+
+public extension Optional {
+    
+    // MARK: Properties
+    
+    /// A Boolean value indicating whether an optional has not been set.
+    var isNil: Bool { self == nil }
+    
+    /// A Boolean value indicating whether an optional has been set.
+    var isNotNil: Bool { !isNil }
+    
+}

Sources/Core/Extensions/String+Empty.swift

@@ -12,17 +12,14 @@
 
 public extension String {
     
-    // MARK: Properties
+    // MARK: Constants
     
-    /// Represents an empty string.
+    /// A string that represents an empty string.
     static let empty = ""
     
-    // MARK: Functions
-    
-    /// Checks whether a string is not empty.
-    /// - Returns: A boolean value that represents whether the string is not empty.
-    func isNotEmpty() -> Bool {
-        isEmpty == false
-    }
+    // MARK: Properties
+
+    /// A Boolean value indicating whether a string is not empty.
+    var isNotEmpty: Bool { !isEmpty }
     
 }

Sources/Core/Extensions/TimeZone+Zone.swift

@@ -4,7 +4,7 @@ public extension TimeZone {
     
     // MARK: Zones
 
-    /// Greenwich Mean Time or UTC+0
+    /// A time zone indicating it is Greenwich Mean Time or UTC+0
     static let gmt = TimeZone(secondsFromGMT: 0)
     
 }