Mocha v10 requires the lib to be esm compliant.
ESM mandates the use of file extensions in imports, so to minimize the
changes (for now), we rely on the flag `experimental-specifier-resolution=node`
and on `ts-node` (needed only for Node 20).
Breaking changes:
downstream bundlers might be affected by the package.json changes depending on
how they load the library.
NB: legacy package.json entrypoints are still available.
