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.

.editorconfig

@@ -0,0 +1,14 @@
+root = true
+
+[*]
+end_of_line = crlf
+insert_final_newline = true
+trim_trailing_whitespace = true
+
+# 4 space indentation
+[*.cs]
+indent_style = space
+indent_size = 4
+
+# CA1820: Test for empty strings using string length
+dotnet_diagnostic.CA1820.severity = error