Revised and patched the workflow for exporting to OpenXml asynchronously

- Added missing DisposeAsync calls in OpenXmlWriter, and implemented IAsyncDisposable interface in SheetStyleBuildContext and EnhancedStreamWriter
- Removed manual duplication of synchronous methods from SheetStyleBuildContext
- Removed superfluous MiniExcelZipArchive class
- Renamed EnhancedStreamWriter to MiniExcelStreamWriter and changed internal StreamWriter initialization to leave underlying stream open to avoid synchronous disposal

Author: michelebastione

src/MiniExcel.Core/Helpers/EnhancedStreamWriter.cs

@@ -0,0 +1,70 @@
+namespace MiniExcelLib.Core.Helpers;
+
+public sealed partial class MiniExcelStreamWriter(Stream stream, Encoding encoding, int bufferSize) : IDisposable
+#if NET8_0_OR_GREATER
+    , IAsyncDisposable
+#endif
+{
+    // if leaveOpen is set to false, the StreamWriter closes the underlying stream synchronously in a finally block.
+    // Since we want to avoid all synchronous operations when dealing with streams we leave it open here, as it will disposed from the caller anyways 
+    private readonly StreamWriter _streamWriter = new(stream, encoding, bufferSize, true);
+    private bool _disposed;
+
+    [CreateSyncVersion]
+    public async Task WriteAsync(string content, CancellationToken cancellationToken = default)
+    {
+        if (!string.IsNullOrEmpty(content))
+        {
+#if NET8_0_OR_GREATER
+            await _streamWriter.WriteAsync(content.AsMemory(), cancellationToken)
+#else
+            cancellationToken.ThrowIfCancellationRequested();
+            await _streamWriter.WriteAsync(content)
+#endif
+                .ConfigureAwait(false);
+        }
+    }
+
+    [CreateSyncVersion]
+    public async Task<long> WriteAndFlushAsync(string content, CancellationToken cancellationToken = default)
+    {
+        await WriteAsync(content, cancellationToken).ConfigureAwait(false);
+        return await FlushAndGetPositionAsync(cancellationToken).ConfigureAwait(false);
+    }
+
+    [CreateSyncVersion]
+    public async Task<long> FlushAndGetPositionAsync(CancellationToken cancellationToken = default)
+    {
+        await _streamWriter.FlushAsync(
+#if NET8_0_OR_GREATER
+            cancellationToken
+#endif
+        ).ConfigureAwait(false);
+        return _streamWriter.BaseStream.Position;
+    }
+
+    public void SetPosition(long position)
+    {
+        _streamWriter.BaseStream.Position = position;
+    }
+
+    public void Dispose()
+    {
+        if (!_disposed)
+        {
+            _streamWriter.Dispose();
+            _disposed = true;
+        }
+    }
+
+#if NET8_0_OR_GREATER
+    public async ValueTask DisposeAsync()
+    {
+        if (!_disposed)
+        {
+            await _streamWriter.DisposeAsync().ConfigureAwait(false);
+            _disposed = true;
+        }
+    }
+#endif
+}