Add a test for explicit context setting.

js/test_cases/explicit_context_setting/README.md

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+It seems $idx and $len do not survive through an explicit context setting, which will work perfectly with my injected-context architecture.

js/test_cases/explicit_context_setting/input1.json

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+{
+  "some_global": "dog",
+  "loop": [
+    {
+      "person": {
+        "name": "Alice"
+      }
+    },
+    {
+      "person": {
+        "name": "Bob"
+      }
+    },
+    {
+      "person": {
+        "name": "Chris"
+      }
+    }
+  ],
+  "explicit": {
+    "pet": "cat"
+  }
+}

js/test_cases/explicit_context_setting/main.dust

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+{! First we do it without explicit context setting which results in being able to read some_global but not pet !}
+{#loop}
+    {#person}
+        {$idx}: {name} has a pet {pet} but not a {some_global}{~n}
+    {/person}
+{/loop}
+
+{! Then we do it with explicit context setting which should result in being able ot read pet but not some_global !}
+{#loop}
+    {#person:explicit}
+        {$idx}: {name} has a pet {pet} but not a {some_global}{~n}
+    {/person}
+{/loop}