feat: add winning condition

[GustavoZiel]

Fixes #44

Checks if snake size has gotten to its maximum for winning condition

Add constants:

• MAXIMUM_SNAKE_SIZE : total number of squares that fit in the arena
• WIN_HEAD_COLOR : snake's head color after winning

Add an attribute to the snake class:

• snake size = 1 (head) + len(tail)

Add an event for test only (removable):

• When pressed 'z' , snake's size attribute is set to its maximum size

Change start point count, from 0 to 1 (undoable)

[monacofj]

Can the snake grow until it occupies all the squares that fit into the arena?
Is this a possible outcome if playing by the rules?

[arthur-qm]

Can the snake grow until it occupies all the squares that fit into the arena? Is this a possible outcome if playing by the rules?

Yes it is. Check #44 (comment) .