Modernize Node support and migrate unit tests to Vitest

[schani]

Summary

• require Node 20 or newer for published npm packages while using Node 24 for development, full CI fixtures, and publishing
• build and run unit tests on both Node 20 and Node 24 in CI
• remove direct fetch polyfills and cover native fetch with local HTTP tests for URL input, remote schema references, and GraphQL introspection
• migrate standalone regression checks to Vitest while retaining the custom cross-language fixture harness
• update the JavaScript PropTypes fixture to use the maintained prop-types API without the legacy fetch dependency chain
• bump the release line to 24.0.0 and add migration documentation
• retain TypeScript 5.8 for builds and TypeScript 4.9 for the TypeScript-to-schema integration

Verification

• clean install with npm ci
• full build on Node 20 and Node 24
• 4 Vitest files / 9 tests on Node 20 and Node 24
• focused JavaScript, TypeScript, and JavaScript PropTypes fixtures
• test TypeScript typecheck and Biome formatting checks
• VS Code extension bundle build
• npm package dry runs for the CLI and all three library packages

The full cross-language matrix remains in GitHub Actions because it requires the external language toolchains described in AGENTS.md.