[Fonts] Add Encoding.GetString(ReadOnlySpan<byte>) polyfill

src/UglyToad.PdfPig.Fonts/Polyfills/EncodingExtensions.cs

@@ -0,0 +1,20 @@
+#if NETFRAMEWORK || NETSTANDARD2_0
+
+namespace System.Text;
+
+internal static class EncodingExtensions
+{
+    public static string GetString(this Encoding encoding, ReadOnlySpan<byte> bytes)
+    {
+        if (bytes.IsEmpty)
+        {
+            return string.Empty;
+        }
+
+        // NOTE: this can be made allocation free by introducing unsafe
+
+        return encoding.GetString(bytes.ToArray());
+    }
+}
+
+#endif