Adds bushes and flowers to ground tiles

Implements bush and flower spawning on ground tiles based on vegetation density.
Adds new assets for bushes and flowers, and introduces multi-mesh rendering for optimized performance.
Introduces seasonal color variations for vegetation using a shader for bushes and materials for flowers and grass.
Refactors material application into a MaterialManager to handle material assignments over multiple frames.
Moves ground tile scripts into a subfolder.
Adds floating particles to test scene.

Utilities/Seasons/seasons.gd

@@ -0,0 +1,11 @@
+class_name SeasonController
+extends Node
+
+var seasons: Array = [
+	"spring",
+	"summer",
+	"autumn",
+	"winter"
+]
+
+@export var current: String = "autumn"

Utilities/Seasons/seasons.gd.uid

@@ -0,0 +1 @@
+uid://buj63lgb7ckto