Video player that has a cross dissolve option when changing the clip
The functionality you seek is standard for a item called a Video Switcher.  Video switchers can be found as both standalone units (low-end to 4K broadcast) and as software (also low-end to high-end).  In the latter case, the computer running the switcher needs video I/O functionality, which typically requires adding one or more special cards (video I/O is different than a graphics card driving a computer monitor).