Set up comprehensive Copilot coding agent instructions

[Copilot]

Expands .github/copilot-instructions.md from a minimal stub into actionable instructions that reflect the actual conventions in this codebase.

Changes

• Testing: Explicitly documents npm run test:nocover as the preferred test command and warns against invoking mocha directly with custom arguments (.mocharc.cjs must be respected)
• Code style: Documents enforced ESLint rules that differ from common defaults — no object shorthand (object-shorthand: never), single quotes, curly braces required, one var per declaration, for...of preferred, no floating promises; notes that strictNullChecks and prefer-const are off
• Imports: import type for type-only imports; external packages before internal modules
• Testing conventions: expect imported from chai-config.spec (not chai directly), sinon sandbox via testHelpers.spec, undent/trim for multiline BrightScript strings, helper functions (expectZeroDiagnostics, etc.)
• Architecture: Expanded key directories (src/bscPlugin/, src/types/, src/validators/) and common patterns (AstEditor for reversible AST edits, DiagnosticMessages for structured error codes, parser error recovery)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>✨ Set up Copilot instructions</issue_title>
<issue_description>Configure instructions for this repository as documented in Best practices for Copilot coding agent in your repository.

</issue_description>

<agent_instructions>look through all of my code to decide my preferred styles. running tests is done with npm run test:nocover. (do not try doing things like mocha --files ....blabla, that won't work).

</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes ✨ Set up Copilot instructions #1649

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