Dispose IDisposables in RegexGenerator (#103142)

* Dispose IDisposables in RegexGenerator

The StringWriter and IndentedTextWriter are IDisposable classes, so we
should dispose them when we are done with them.

* Update src/libraries/System.Text.RegularExpressions/gen/RegexGenerator.cs

Co-authored-by: Günther Foidl <gue@korporal.at>

---------

Co-authored-by: Buyaa Namnan <buyankhishig.namnan@microsoft.com>
Co-authored-by: Günther Foidl <gue@korporal.at>

Author: 3 people

src/libraries/System.Text.RegularExpressions/gen/RegexGenerator.cs

@@ -140,8 +140,8 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
                 }
 
                 // At this point we'll be emitting code.  Create a writer to hold it all.
-                var sw = new StringWriter();
-                IndentedTextWriter writer = new(sw);
+                using StringWriter sw = new();
+                using IndentedTextWriter writer = new(sw);
 
                 // Add file headers and required usings.
                 foreach (string header in s_headers)