0.9.89.6b Improved aircraft handling

0.9.89.6b • Improved aircraft handling and added the same engine boost feature used by vehicles. Hold the boost key to increase speed. • Added sirens to the Stuka. It will now let out its signature wail when diving. • Fixed a bug where enemies would acquire cutting-edge invisibility technology after entering or exiting an aircraft. • Fixed aircraft combat duration not scaling correctly with the turret card's Range stat. • Nora's character artwork and the BlackAdder battle theme are now located in the game's root directory: Multi Turret Academy_Data\StreamingAssets