Enemies can now spawn with different speeds

-Enemies have increased movement speed based on the level
-Lowered the max spawn time available and increased the amount the min spawn time decreases each level
Improved spawn balance

-Levels initially start out shorter but get longer
-At level 50, the max level, enemies no longer move ridiculously fast and spawn at more reasonable times
-Added a new enemy type, Armored Spear Goblin, and accompanying spritesheets
-Added a recolored sprite of the Player Castle that is used after the player purchases the new FortifyCastle ShopItem
  -This offers 10% damage reduction but can only be bought once and is quite costly
-Made all weapons have the same cooldown values since there is already a gold penalty for using stronger weapons against weaker enemies
-Made the ThrowSpear action more flexible to account for the ArmoredSpearEnemy
Created the Slower Player Helper

-The Slower slows a random enemy on the screen and has infinite range
-Added MinimumMoveSpeed, SlowAmount, and SlowTime fields to the Enemy.cs for slow debuff support
-Added the ApplySlow and CheckEndSlow methods to Enemy.cs
-Created Slower.cs, which is similar to Archer.cs
-Made the Sword weapon get selected when a new game is started or an existing game is continued
Created the Invincibility Shop Item

-The player can only buy one Invincibility item at a time, and it is a one-time use item
-Added a UseItemInGame method to ShopItem.cs so that consumables have a method that activates their in-game effects
-Added a reference to the Shop in the base ShopItem.cs class and all derived Shop Items. Changed the constructor of the base ShopItem.cs class and every ShopItem accordingly to account for this
-Made the player able to click/tap on consumables on the HUD to use them. After a consumable is used, it is removed from the HUD
-Added the FortifyCastle upgrade and the Invincibility item to the Shop
Made a Visual Effect Appear on Enemies When Slowed

-Slowed enemies will have a light coral color overlay
Changed the Color of the Slow Effect on Enemies

-Changed the slow effect color from LightCoral to LightPink
-Changed IsSlowed to a property instead of a field in Enemy.cs
-Removed void CheckEndSlow() from Enemy.cs because it was no longer needed
-Decreased the enemy's movement speed if the enemy is slowed in the GetMoveSpeed property
Created an animation for the Slower
Merge branch 'SlowerAnimation' into SlowerVisualEffectImprovement
Merge branch 'SlowerVisualEffectImprovement'