Convert to ESLint 9 flat config and move to pnpm, use .mjs and new jest types

[SleeplessByte]

Related: https://github.com/exercism/javascript-test-runner/pull/852

Similar to: https://github.com/exercism/typescript/pull/1506

• Migrate to pnpm: means less space used, and installing can often be done without internet connection as long as you download the exercise first.
• Upgrade to ESLint 9
• Upgrade SDKs
• Sync
• Format files
• Fix .vscode files, upgrade all tests to new format
• Update workflows to Node LTS @ 20
• Fix lint issues
• Remove babel transpilation for common actions
• Simplify changed files check