Created "Engineer" based on Issue #218

[JerryLin828]

Introduction

Based on Issue #218, I created 2 units, "Engineer" and "Small Turret", which function as described below. This is built on my first PR, #224, where I implemented the "Double Shrinker".

Engineer

• Appearance: Unlike a tank, the engineer is a single person. The icon is inspired by the classic game surviv.io: a large circle represents the person's head, while two small circles represent their hands. The head is painted red for better distinction.
• Moving: Since the engineer does not have a turret, there is no need for rotation. Therefore, the keys W, S, A, and D are used to move the engineer upwards, downwards, leftwards, and rightwards, respectively.
• Health: As the engineer is only human, their maximum health is limited to 20 points.
• Creating Small Turrets: The engineer has a special skill that allows them to build small turrets (half the size of tank turrets) at their current position. After each building action, the skill must cool down for 5 seconds (fire_count_down = 5s) before it is ready to be used again.

Small Turret

• Appearance: The small turret looks identical to a tank turret but is half its size.
• Rotating: The player cannot manually rotate the turret. Instead, the turret automatically identifies the nearest enemy (another player) and points in their direction.
• Shooting: Similar to the tiny_tank, the turret has a fire_count_down of 1 second, and it shoots a regular cannonball (normal bullet as described in Create an "engineer" unit that places attackable obstacles #218). The player can press P to fire.
• Health: The turret has a maximum health of 100 points. When it is hit, the cannonball disappears.
• Disappearance: The small turret disappears 10 seconds after being built. As a result, there can be at most 2 small turrets in the area at any given time.

You can see how they function in the video below:

Engineer.Video.mov

Closes #218