game: add weapon mod system

scenes/weapons/weapon_mod_base.gd

@@ -0,0 +1,36 @@
+class_name WeaponModBase
+extends Resource
+
+enum ModType { ADDITIVE, MULTIPLICATIVE, ABSOLUTE, BOOL }
+
+@export var name: String = "Unnamed mod"
+@export var description: String = ""
+@export var icon: Texture2D
+@export var mod_property: String
+@export var mod_value: float
+@export var mod_value_bool: bool = false
+@export var mod_type: ModType = ModType.MULTIPLICATIVE
+@export var mod_extra_tags: Array[WeaponBase.WeaponTag] = []
+
+
+static func get_calculated(weapon: WeaponBase, key: String) -> Variant:
+	assert(
+		weapon.has_property(key),
+		"tried calculate property '%s' where base value does not exist on %s" % [key, weapon]
+	)
+	var base_value = weapon.get(key)
+
+	var add = 0.0
+	var mul = 1.0
+	for mod in weapon.modifiers:
+		if mod.mod_property == key:
+			match mod.mod_type:
+				ModType.ADDITIVE:
+					add += mod.mod_value
+				ModType.MULTIPLICATIVE:
+					mul *= mod.mod_value
+				ModType.ABSOLUTE:
+					return mod.mod_value
+				ModType.BOOL:
+					return mod.mod_value_bool
+	return (base_value + add) * mul