This PR creates a comprehensive guide for writing TypeScript tests to help Copilot understand how to write fourslash tests and compiler tests.

What's included

Created .github/copilot-instructions.md containing:

Build Instructions Summary

• Setup instructions with hereby command line tool
• Common build tasks (hereby local, hereby tests, hereby runtests, etc.)
• Test running patterns for specific tests and runners

Fourslash Test Syntax Guide

• Basic structure with /// <reference path='fourslash.ts'/>
• Source code definition using ////
• Markers with /**/ and /*name*/ syntax for positioning
• Multi-file tests with // @Filename:
• Text ranges using [|text|]
• Common API patterns focusing on validation over baselines:

  verify.currentLineContentIs("expected content");
  verify.completions({ includes: "itemName" });
  verify.codeFix({ description: "Fix description", newFileContent: "..." });

Compiler Test Syntax Guide

• Simple .ts file structure in tests/cases/compiler/
• Compiler directives using // @directive: value
• Multi-file test patterns
• Error expectation documentation with comments
• Type testing patterns

Examples

Fourslash test example:

/// <reference path='fourslash.ts'/>

////interface User {
////    name: string;
////}
////
////const user: User = {
////    /*completion*/
////};

verify.completions({
    marker: "completion",
    includes: { name: "name", sortText: completion.SortText.LocationPriority }
});

Compiler test example:

// @strict: true

interface Config {
    required: string;
    optional?: number;
}

const config1: Config = { required: "test" }; // Should work
const config2: Config = { optional: 42 }; // Should error - missing required

Focus on simplicity

The guide prioritizes:

• Simplest forms of each syntax pattern
• Validation over baselines for fourslash tests
• Clear, focused examples that test one feature at a time
• Realistic scenarios developers actually encounter

The documentation has been tested against the actual codebase to ensure accuracy.

Fixes #61899.

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