Update on terrain manager system

[igorfel]

• Added a scriptable object to handle the environment tiles and the procedural generation function;
• Added a feature to use tilemaps as a disabled area: The procedural function receives an array of tilemaps and for each cell it checks if one of the tilemaps has a tile on that specific cell, and if not, it draws a tile for the environment depending on the density;
• Added a feature to control the density [Range[0f,1f)]; being 1 to draw on every available cell and 0 to not draw anything.

[rsaz]

Thank you for your contribution