This pull request adds initial support for C# syntax highlighting by introducing a new C# language definition and an associated test file. It also updates the highlighting engine to treat C# preprocessor directives with a specific highlight color.

C# syntax highlighting support:

• Added a new C# language definition in csharp.lsh, covering preprocessor directives, comments, various string and character literal forms, attributes, keywords, numeric literals, and method names.
• Introduced a comprehensive C# test file (csharp.cs) to validate the new syntax highlighting rules.

Highlighting engine improvements:

• Updated the buffer implementation to highlight C# preprocessor directives (meta.preprocessor) using the BrightBlue color.

Screenshot