Add StringBuilder Insert (#282)

Author: SimonCropp

src/Polyfill/Polyfill_StringBuilder_Insert.cs

@@ -0,0 +1,22 @@
+// <auto-generated />
+#pragma warning disable
+
+namespace Polyfills;
+
+using System;
+using System.Text;
+
+static partial class Polyfill
+{
+#if (!NETCOREAPP2_1_OR_GREATER || NETSTANDARD2_0) && FeatureMemory
+    /// <summary>
+    /// Inserts the sequence of characters into this instance at the specified character position.
+    /// </summary>
+    /// <param name="index">The position in this instance where insertion begins.</param>
+    /// <param name="value">The character span to insert.</param>
+    /// <returns>A reference to this instance after the insert operation has completed.</returns>
+    //Link: https://learn.microsoft.com/en-us/dotnet/api/system.text.stringbuilder.insert#system-text-stringbuilder-insert(system-int32-system-readonlyspan((system-char)))
+    public static StringBuilder Insert(this StringBuilder target, int index, ReadOnlySpan<char> value) =>
+        target.Insert(index, value.ToString());
+#endif
+}