Improve Copilot instructions and add test guidelines

- Enhanced copilot instructions with detailed architecture overview
- Added comprehensive development workflow documentation
- Documented addon development patterns and key examples
- Added project-specific conventions and TypeScript structure
- Included common patterns for parser integration and buffer access
- Added critical implementation details for rendering and events
- Created unit test instructions file with addon testing rules
- Specified preferences for assertions and self-documenting tests

.github/instructions/unit-test-instructions.instructions.md

@@ -0,0 +1,8 @@
+---
+applyTo: '**/*.test.ts'
+---
+When writing unit tests follow these rules:
+
+- When writing unit tests for addons, always create a real xterm.js instance instead of mocking it.
+- Prefer `assert.ok` over `assert.notStrictEqual` when checking something is undefined or not.
+- Avoid comments as most tests should be self-documenting.