Add missing textures and models from Chorus' Version

This also helps in preparing for the Nether Update (1.16). Textures were
optimised using OptiPNG by:

optipng -o7 -nx

followed by a second run with the defaults.

assets/minecraft/textures/blockstates/sand.json

@@ -0,0 +1,10 @@
+{
+    "variants": {
+        "": [
+            { "model": "block/sand" },
+            { "model": "block/sand", "y": 90 },
+            { "model": "block/sand", "y": 180 },
+            { "model": "block/sand", "y": 90 }
+        ]
+    }
+}