Initial commit of the ClosedXML library

Establishes the foundational structure, core Excel functionality, and tooling.

- Implements base components for cells, ranges, columns, and rows.
- Integrates a custom calculation engine with a wide range of Excel functions.
- Adds comprehensive support for styling, conditional formatting, data validation, comments, pictures, and charts.
- Configures build setup, project metadata (NuGet), and developer guidelines.
- Includes editor and Git attributes for consistent code style and line endings.

appveyor.yml

@@ -0,0 +1,61 @@
+version: 0.95.4.{build}
+
+os: Visual Studio 2019
+image: Visual Studio 2019
+
+environment:
+  AppVeyor: APPVEYOR
+
+init:
+
+- ps: >-
+    if ($env:APPVEYOR_REPO_TAG -eq 'true')
+    {
+      $env:fileVersion = $env:APPVEYOR_REPO_TAG_NAME -replace '(\d+)\.(\d+)\.(\d+)(-.+)?', '$1.$2.$3'
+      if ($env:fileVersion -eq $env:APPVEYOR_REPO_TAG_NAME) { $env:fileVersion = $($env:fileVersion + '.0') }
+      else { $env:fileVersion = $($env:fileVersion + '.' + $env:APPVEYOR_BUILD_NUMBER) }
+      $env:productVersion = $env:APPVEYOR_REPO_TAG_NAME
+    }
+    else
+    {
+      $env:fileVersion = $env:APPVEYOR_BUILD_VERSION -replace '(\d+)\.(\d+)\.([^.]+)\.(\d+)', '$1.$2.999.$4'
+      $env:productVersion = $env:fileVersion
+    }
+    
+    Update-AppveyorBuild -Version $env:fileVersion
+    
+    Write-Host $env:fileVersion $env:productVersion
+    
+    Write-Host $env:APPVEYOR_REPO_TAG $env:APPVEYOR_REPO_TAG_NAME
+
+
+dotnet_csproj:
+  patch: true
+  file: '**\*.csproj'
+  version: '$(productVersion)'
+  package_version: '$(productVersion)'
+  assembly_version: '$(fileVersion)'
+  file_version: '$(fileVersion)'
+  informational_version: '$(productVersion)'
+
+# platform: Any CPU
+configuration : 
+- Release
+- Release.Signed
+
+before_build:
+  - cmd: nuget update -self
+  - nuget restore
+
+build:
+  parallel: true
+  project: ClosedXML.sln
+  verbosity: minimal
+
+artifacts:
+  - path: ClosedXML/bin/%CONFIGURATION%/*/ClosedXML.dll
+  - path: ClosedXML/bin/%CONFIGURATION%/*.nupkg
+
+nuget:
+  project_feed: true
+  disable_publish_on_pr: true