Move non-jellyfin extensions to separate project

src/Jellyfin.Extensions/StringExtensions.cs

@@ -0,0 +1,31 @@
+using System;
+
+namespace Jellyfin.Extensions
+{
+    /// <summary>
+    /// Provides extensions methods for <see cref="string" />.
+    /// </summary>
+    public static class StringExtensions
+    {
+        /// <summary>
+        /// Counts the number of occurrences of [needle] in the string.
+        /// </summary>
+        /// <param name="value">The haystack to search in.</param>
+        /// <param name="needle">The character to search for.</param>
+        /// <returns>The number of occurrences of the [needle] character.</returns>
+        public static int Count(this ReadOnlySpan<char> value, char needle)
+        {
+            var count = 0;
+            var length = value.Length;
+            for (var i = 0; i < length; i++)
+            {
+                if (value[i] == needle)
+                {
+                    count++;
+                }
+            }
+
+            return count;
+        }
+    }
+}