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/models/block/bricks/halfslab/11.json

@@ -0,0 +1,9 @@
+{
+    "__comment": "Fair warning, this format is highly likely to change even more in the future!",
+    "parent": "block/upper_slab",
+    "textures": {
+        "bottom": "blocks/brick4",
+        "top": "blocks/brick4",
+        "side": "blocks/brick4"
+    }
+}