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.

.gitattributes

@@ -0,0 +1,21 @@
+# Source https://help.github.com/articles/dealing-with-line-endings
+# Set default behaviour, in case users don't have core.autocrlf set.
+* text=auto
+
+# Explicitly declare text files we want to always be normalized and converted 
+# to native line endings on checkout.
+*.cs text diff=csharp eol=crlf
+
+# Declare files that will always have CRLF line endings on checkout.
+*.sln text eol=crlf
+*.csproj text eol=crlf
+
+# Denote all files that are truly binary and should not be modified.
+*.png binary
+*.jpg binary
+*.dll binary
+*.gif binary
+*.ico binary
+*.xlsx binary
+
+core.autocrlf=true