Add universal authoring guidelines for consistent code formatting and structure

Author: MariusStorhaug

.github/instructions/universal.instructions.md

@@ -0,0 +1,13 @@
+---
+applyTo: '**'
+description: Standardize and improve code with consistent formatting, structure, and clarity.
+---
+
+## Universal Authoring Guidelines
+
+Lines SHALL not exceed 150 characters.
+Lines SHALL not have trailing whitespace at the end of lines, including blank lines.
+All files MUST end with a single newline.
+Paths SHALL use forward slashes (`/`), not backslashes (`\`) for compatibility across operating systems.
+Files SHALL use UTF-8 encoding.
+Files SHALL use Unix line endings (LF).