feat: 텍스트 문서 5종 양방향 변환 매트릭스 (DocumentProvider)

신규 DocumentProvider — html/htm/hwp/hwpx/docx/doc/md/markdown/txt 입력 →
html/docx/md/txt 출력 (HWP 출력은 H2Orestart 한계로 미지원).

라우팅 (멀티홉)
- {docx, html, txt, hwp} ↔ {docx, html, txt}: LibreOffice --convert-to
- md → html: Markdig (advanced extensions)
- html → md: ReverseMarkdown (GitHub flavored)
- md → docx/txt: md → html(Markdig) → LibreOffice 체인
- {docx, html, hwp} → md: → html(LibreOffice) → md(ReverseMarkdown) 체인
- md ↔ txt, html ↔ html 등: trivial copy
- txt → html: pre-wrapped HTML 인코딩

NuGet 추가
- Markdig 0.37.0
- ReverseMarkdown 4.6.0 (HtmlAgilityPack 의존성 자동 포함)

UI/메뉴 갱신
- AllFormats에 .html, .md 추가 (총 12종)
- PopularOutputs 카스케이드 12개로 확장:
  JPEG/PNG/WebP/PDF/Word/HTML/MD/TXT/AVIF/GIF/TIFF/BMP

요구 외부 도구: LibreOffice (DocxProvider/HwpxProvider와 공통)
MSIX 갱신: Markdig.dll + ReverseMarkdown.dll + HtmlAgilityPack.dll 포함

src/Everything2Everything.Core/Everything2Everything.Core.csproj

@@ -18,6 +18,8 @@
     <PackageReference Include="PhotoSauce.NativeCodecs.Libheif" Version="1.19.5-preview1" />
     <PackageReference Include="Microsoft.Web.WebView2" Version="1.0.3912.50" />
     <PackageReference Include="DocumentFormat.OpenXml" Version="3.1.0" />
+    <PackageReference Include="Markdig" Version="0.37.0" />
+    <PackageReference Include="ReverseMarkdown" Version="4.6.0" />
   </ItemGroup>
 
   <ItemGroup>