Remove StringBuilderCache usage from System.Text.RegularExpressions (#30474)

* Remove StringBuilderCache usage

* fixed buffer size, manual reverse copying & local textinfo

* Fast path for not to encode string

* avoid one span slice

* code formatting & bounds check opt

* Avoid string allocation in char class

Signed-off-by: dotnet-bot-corefx-mirror <dotnet-bot@microsoft.com>

Author: ViktorHofer

src/System.Private.CoreLib/shared/System/Text/ValueStringBuilder.cs

@@ -9,7 +9,7 @@
 
 namespace System.Text
 {
-    internal ref struct ValueStringBuilder
+    internal ref partial struct ValueStringBuilder
     {
         private char[] _arrayToReturnToPool;
         private Span<char> _chars;