v3.1.0: Adjust peer dependencies; convert Babel configuration to ESM

🌎 External changes

• Add optional peer dependencies @vitest/coverage-istanbul, core-js, and vitest.
• Mark as optional formerly required peer dependencies @babel/core, @babel/plugin-syntax-import-attributes, @babel/preset-env, and @babel/preset-typescript.
• Drop orphaned peer dependency @babel/eslint-parser.

🏠 Internal changes

• Convert Babel configuration from CommonJS to ESM and delete the internal proxy file in favor of babel.extends in package.json.
• Upgrade dependencies.