Add copilot instructions

Author: nohwnd

.github/copilot-instructions.md

@@ -0,0 +1,30 @@
+This is a .NET based repository that contains the VSTest test platform. Please follow these guidelines when contributing:
+
+## Code Standards
+
+You MUST follow all code-formatting and naming conventions defined in [`.editorconfig`](../.editorconfig).
+
+In addition to the rules enforced by `.editorconfig`, you SHOULD:
+
+- Favor style and conventions that are consistent with the existing codebase.
+- Prefer file-scoped namespace declarations and single-line using directives.
+- Ensure that the final return statement of a method is on its own line.
+- Use pattern matching and switch expressions wherever possible.
+- Use `nameof` instead of string literals when referring to member names.
+- Always use `is null` or `is not null` instead of `== null` or `!= null`.
+- Trust the C# null annotations and don't add null checks when the type system says a value cannot be null.
+- Prefer `?.` if applicable (e.g. `scope?.Dispose()`).
+- Use `ObjectDisposedException.ThrowIf` where applicable.
+- Respect StyleCop.Analyzers rules, in particular:
+  - SA1028: Code must not contain trailing whitespace
+  - SA1316: Tuple element names should use correct casing
+  - SA1518: File is required to end with a single newline character
+
+You MUST minimize adding public API surface area but any newly added public API MUST be declared in the related `PublicAPI.Unshipped.txt` file.
+
+## Localization Guidelines
+
+Anytime you add a new localization resource, you MUST:
+- Add a corresponding entry in the localization resource file.
+- Add an entry in all `*.xlf` files related to the modified `.resx` file.
+- Do not modify existing entries in '*.xlf' files unless you are also modifying the corresponding `.resx` file.