introduced 'node' engine minimal requirement in root package.json sta…

…ting that building Langium and running tests requires Node.js v18;

* minimally required version of Node.js for Langium at runtime is still v16, as stated in 'packages/langium/package.json'
* bumped version of vitest to "~1.0.0",
* added version override on '@types/node' to cope with vitest's (transitive) peer dependency on '@types/node@"^18.0.0 || >=20.0.0"'
* updated '@types/node' version pinned in 'package-lock.json' to the latest release (16.18.68)
* renamed 'vite.config.ts' to 'vite.config.mts' as Vite/Vitest is going to drop the CJS support for it's configuration API