fix timestamp creation for scoped pkgs

[mmomtchev]

Timestamp creation fails on macOS and Linux for scoped pkgs which contain a / as part of the package name

Replace path.sep with -

[ilg-ul]

Thank you for the contribution, but the migration of the cli-start-options package from JS to TS will require some more work.

The first version of the project was written in JS, and, at a certain moment, I tried to rewrite it in TS, as a first step to rewrite the whole xpm project in TS and migrate to ESM.

Unfortunately I soon discovered that this is not such a good idea, since xpm is also used in the VS Code extension, which was not able to import ESM, so I postponed everything.

Apparently VS Code was updated and is, or will be very soon, able to import ESM, so I'll be able to modernise everything.