✨ Add string.IsUpper and string.IsLower

- IsLower:
Determines if all alpha characters in this string are considered
lowercase.

- IsUpper:
Determines if all alpha characters in this string are considered
uppercase.

CHANGELOG.md

@@ -4,6 +4,10 @@
 ### Added
 - Add `string.ChangeEncoding(Encoding, Encoding)`
     - Converts this string from one encoding to another
+- Add `string.IsLower`
+    - Determines if all alpha characters in this string are considered lowercase
+- Add `string.IsUpper`
+    - Determines if all alpha characters in this string are considered uppercase
 
 ### Changed
 - n/a

X10D/src/StringExtensions.cs

@@ -145,6 +145,28 @@
             return encoding.GetBytes(str);
         }
 
+        /// <summary>
+        /// Determines if all alpha characters in this string are considered lowercase.
+        /// </summary>
+        /// <param name="str">The input string.</param>
+        /// <returns>Returns <see langword="true"/> if all alpha characters are lowercase, <see langword="false"/>
+        /// otherwise.</returns>
+        public static bool IsLower(this string str)
+        {
+            return str.Where(char.IsLetter).All(char.IsLower);
+        }
+
+        /// <summary>
+        /// Determines if all alpha characters in this string are considered uppercase.
+        /// </summary>
+        /// <param name="str">The input string.</param>
+        /// <returns>Returns <see langword="true"/> if all alpha characters are uppercase, <see langword="false"/>
+        /// otherwise.</returns>
+        public static bool IsUpper(this string str)
+        {
+            return str.Where(char.IsLetter).All(char.IsUpper);
+        }
+
         /// <summary>
         /// Generates a new random string by filling it with characters found in <see cref="str"/>.
         /// </summary>