[BUG] Item with "path like" name gets stripped when exporting

Having an item that has some "path like" components like `hello/world` gets exported as `world`.

This is due to us treating the item name as a "path" when we want to remove a potential extension from the name:
https://github.com/graasp/graasp/blob/4428861f7eb2eea6c83eca111cac7070c84a0d42/src/services/item/plugins/importExport/utils.ts#L58

Treating the item name as a path can have unexpected consequences, and we should probably use another technique to remove a potential extension.