refactor: Spread game code into modular files for better maintainability

This commit reorganizes the game's source code into multiple files within a `js/` directory, creating a more modular and maintainable structure. The changes include:

- Created separate files for different game components:
  - `constants.js`: Game constants and element types
  - `world.js`: World management functions
  - `terrain.js`: Terrain generation logic
  - `physics.js`: Physics simulation
  - `render.js`: Rendering functions
  - `input.js`: Input handling
  - `main.js`: Main game initialization and loop
  - Element-specific files in `js/elements/`:
    - `basic.js`: Sand, water, dirt behaviors
    - `plants.js`: Grass, seeds, flowers
    - `trees.js`: Tree growth and leaf generation
    - `fire.js`: Fire and lava behaviors

- Updated `index.html` to load modules in the correct order
- Removed the monolithic `script.js`

The modular approach improves code readability, makes future extensions easier, and separates concerns more effectively.

js/constants.js

@@ -0,0 +1,39 @@
+// Game constants
+const CHUNK_SIZE = 200;
+const PIXEL_SIZE = 4;
+const GRAVITY = 0.5;
+const WATER_SPREAD = 3;
+
+// Colors
+const SAND_COLOR = '#e6c588';
+const WATER_COLOR = '#4a80f5';
+const WALL_COLOR = '#888888';
+const DIRT_COLOR = '#8B4513';
+const STONE_COLOR = '#A9A9A9';
+const GRASS_COLOR = '#7CFC00';
+const WOOD_COLOR = '#8B5A2B';
+const SEED_COLOR = '#654321';
+const FLOWER_COLORS = ['#FF0000', '#FFFF00', '#FF00FF', '#FFA500', '#FFFFFF', '#00FFFF'];
+const LEAF_COLOR = '#228B22';
+const FIRE_COLORS = ['#FF0000', '#FF3300', '#FF6600', '#FF9900', '#FFCC00', '#FFFF00'];
+const LAVA_COLORS = ['#FF0000', '#FF3300', '#FF4500', '#FF6600', '#FF8C00'];
+
+// Element types
+const EMPTY = 0;
+const SAND = 1;
+const WATER = 2;
+const WALL = 3;
+const DIRT = 4;
+const STONE = 5;
+const GRASS = 6;
+const WOOD = 7;
+const SEED = 8;
+const GRASS_BLADE = 9;
+const FLOWER = 10;
+const TREE_SEED = 11;
+const LEAF = 12;
+const FIRE = 13;
+const LAVA = 14;
+
+// Flammable materials
+const FLAMMABLE_MATERIALS = [GRASS, WOOD, SEED, GRASS_BLADE, FLOWER, TREE_SEED, LEAF];