Migrate to ESLint 9 flat config, replace vscode-test with @vscode/test-electron, and add GitHub Actions CI

[Copilot]

Summary

This PR migrates the project to ESLint 9 with flat config, replaces the deprecated vscode-test package with @vscode/test-electron, and adds a GitHub Actions CI workflow.

Changes

ESLint 9 migration

• Updated eslint from ^8.6.0 to ^9.0.0
• Updated @typescript-eslint/eslint-plugin and @typescript-eslint/parser from ^5.9.0 to ^8.58.0
• Added typescript-eslint ^8.58.0 (new umbrella package for flat config API)
• Created eslint.config.mjs (ESLint 9 flat config) with equivalent rules to the old config
• Deleted .eslintrc.json and .eslintignore (replaced by eslint.config.mjs)
• Updated lint script: removed --config .eslintrc.json, added --no-warn-ignored

vscode-test replacement

• Replaced vscode-test ^1.3.0 with @vscode/test-electron ^2.5.2 in devDependencies
• Updated test/test-integration/runTest.ts import from vscode-test to @vscode/test-electron

GitHub Actions CI

• Added .github/workflows/ci.yml that runs on pushes and pull requests targeting main
• Workflow steps: install dependencies (npm ci), build (npm run esbuild), lint (npm run lint), TypeScript compile (npm run ts-compile), and unit tests (npm run unit-test)
• Integration tests are excluded from this workflow as they require a VS Code/Xvfb environment (handled by the existing Azure Pipelines setup)

[rzhao271]

@copilot Add a new GitHub Actions CI that runs install build test (if applicable) and lint checks.

[Copilot]

@copilot Add a new GitHub Actions CI that runs install build test (if applicable) and lint checks.

Added in commit 5f5ba37. The workflow (.github/workflows/ci.yml) runs on pushes and PRs to main and performs:

1. Install – npm ci
2. Build – npm run esbuild
3. Lint – npm run lint
4. TypeScript compile – npm run ts-compile
5. Unit tests – npm run unit-test

Integration tests are omitted since they require a running VS Code instance (Xvfb + special env), which matches the existing Azure Pipelines setup for that step.

[rzhao271]

@copilot Reapply changes to resolve merge conflict

The merge-base changed after approval.

[Copilot]

@copilot Reapply changes to resolve merge conflict

Done in commit e3626b0. Merged origin/main (the lodash-es bump) into the branch and resolved the package-lock.json conflict — our lock file already had lodash-es 4.18.1 with the newer npm v3 format (no legacy "dependencies" section), so that was kept.