vspreview: make it usable with `python3 -m vspreview`

[AkarinVS]

By renaming main.py to main.py, we can directly use python3 -m vspreview to start it, without using run.py.

Of course, this rename might cause some merging issues for forks, so an alternative and minimum change is to just create a new __main__.py with the following content:

from .main import main
main()

Please let me know your preferences.

[Endilll]

Thank you for nice suggestion. I'd prefer keeping main.py untouched.

[AkarinVS]

PTAL.