New builds and an intro Level 0

It's amazing that I got some feedback already. Thank you very much for that! In response, here are the changes that have been made to the game:

• Space Bomber now starts at Level 0  with a limited number of enemies and powerup pillars. It is smaller Level 1, which should let players understand the basic mechanics of the game before having to face dozens of aliens on a larger map
• I tweaked the bomb aftershock duration and collider of the player to make it less easy to simply walk into an exploding bomb and die
• I added the Windows and Linux executables so you can enjoy the game on those platforms as well

I still have more ideas for making this experiment a more viable game (e.g. bosses, a true ending) and I welcome all the feedback I can get.