Migrate to @vscode/test-cli for modern test infrastructure

[pete-the-pete]

Description

When running npm run test on master, I got errors about the vscode instance (which may have been b/c of a stale version on my machine), but even after fixing that I got a bunch of failures from a missing .env.

Proposed Changes

• Add .vscode-test.mjs configuration file using @vscode/test-cli API
• Update test script in package.json to use vscode-test command
• Upgrade @vscode/test-electron from ^2.1.5 to ^2.5.2
• Add @vscode/test-cli (^0.0.12) as dev dependency
• Add Mocha environment configuration for ESLint compatibility
• Optimize runTest.js to reuse local VS Code installation

Benefits

• Cleaner, more maintainable test configuration
• Faster test execution (reuses existing VS Code install)
• Better alignment with official VS Code extension development guidelines
• Simplified test command (vscode-test instead of custom runner)