Add back command computer block drops

This has been broken for almost a year (28th Jan 2020), and I never
noticed. Good job me.

Fixes #641, closes #648 (basically the same, but targetting 1.15.x)

src/generated/resources/data/computercraft/loot_tables/blocks/computer_command.json

@@ -0,0 +1,34 @@
+{
+  "type": "minecraft:block",
+  "pools": [
+    {
+      "name": "main",
+      "rolls": 1,
+      "entries": [
+        {
+          "type": "minecraft:dynamic",
+          "name": "computercraft:computer"
+        }
+      ],
+      "conditions": [
+        {
+          "condition": "minecraft:alternative",
+          "terms": [
+            {
+              "condition": "computercraft:block_named"
+            },
+            {
+              "condition": "computercraft:has_id"
+            },
+            {
+              "condition": "minecraft:inverted",
+              "term": {
+                "condition": "computercraft:player_creative"
+              }
+            }
+          ]
+        }
+      ]
+    }
+  ]
+}

src/main/java/dan200/computercraft/data/LootTables.java

@@ -42,6 +42,7 @@ protected void registerLoot( BiConsumer<ResourceLocation, LootTable> add )
 
         computerDrop( add, Registry.ModBlocks.COMPUTER_NORMAL );
         computerDrop( add, Registry.ModBlocks.COMPUTER_ADVANCED );
+        computerDrop( add, Registry.ModBlocks.COMPUTER_COMMAND );
         computerDrop( add, Registry.ModBlocks.TURTLE_NORMAL );
         computerDrop( add, Registry.ModBlocks.TURTLE_ADVANCED );