Add StandardCharsets support class and update TestBinaryDocument test…

… to use that

src/Lucene.Net.Tests/Document/TestBinaryDocument.cs

@@ -1,4 +1,5 @@
 using J2N.Text;
+using Lucene.Net.Support;
 using NUnit.Framework;
 using System;
 using System.Text;
@@ -108,7 +109,7 @@ public virtual void TestCompressionTools()
             // fetch the binary compressed field and compare it's content with the original one
             // LUCENENET: was `= new String(CompressionTools.decompress(docFromReader.getBinaryValue("binaryCompressed")), StandardCharsets.UTF_8);`
             string binaryFldCompressedTest =
-                Encoding.UTF8.GetString(
+                StandardCharsets.UTF_8.GetString(
                     CompressionTools.Decompress(docFromReader.GetBinaryValue("binaryCompressed")));
             Assert.IsTrue(binaryFldCompressedTest.Equals(binaryValCompressed, StringComparison.Ordinal));
             Assert.IsTrue(

src/Lucene.Net/Support/StandardCharsets.cs

@@ -0,0 +1,26 @@
+/*
+* Licensed to the Apache Software Foundation (ASF) under one or more
+* contributor license agreements.  See the NOTICE file distributed with
+* this work for additional information regarding copyright ownership.
+* The ASF licenses this file to You under the Apache License, Version 2.0
+* (the "License"); you may not use this file except in compliance with
+* the License.  You may obtain a copy of the License at
+*
+*     http://www.apache.org/licenses/LICENSE-2.0
+*
+* Unless required by applicable law or agreed to in writing, software
+* distributed under the License is distributed on an "AS IS" BASIS,
+* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+* See the License for the specific language governing permissions and
+* limitations under the License.
+*/
+
+using Lucene.Net.Util;
+using System.Text;
+
+namespace Lucene.Net.Support;
+
+internal static class StandardCharsets
+{
+    public static readonly Encoding UTF_8 = IOUtils.CHARSET_UTF_8;
+}