Add a test for $idx and $len.

js/test_cases/idx_and_len/README.md

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+$idx and $len seem to only be valid inside sections iterating over arrays, but nothing else.

js/test_cases/idx_and_len/array.json

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+{
+  "things": [
+    "Alice",
+    "Bob",
+    "Chris"
+  ]
+}

js/test_cases/idx_and_len/false.json

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+{
+  "things": false
+}

js/test_cases/idx_and_len/main.dust

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+Outside $idx: {$idx}{~n}
+Outside $len: {$len}{~n}
+Outside {?$idx}$idx is true{:else}$idx is false{/$idx}{~n}
+Outside {?$len}$len is true{:else}$len is false{/$len}{~n}
+
+{#things}
+    Inside Section $idx: {$idx}{~n}
+    Inside Section $len: {$len}{~n}
+    Inside Section {?$idx}$idx is true{:else}$idx is false{/$idx}{~n}
+    Inside Section {?$len}$len is true{:else}$len is false{/$len}{~n}
+{:else}
+    Else Section $idx: {$idx}{~n}
+    Else Section $len: {$len}{~n}
+    Else Section {?$idx}$idx is true{:else}$idx is false{/$idx}{~n}
+    Else Section {?$len}$len is true{:else}$len is false{/$len}{~n}
+{/things}
+
+{?things}
+    Inside Exists $idx: {$idx}{~n}
+    Inside Exists $len: {$len}{~n}
+    Inside Exists {?$idx}$idx is true{:else}$idx is false{/$idx}{~n}
+    Inside Exists {?$len}$len is true{:else}$len is false{/$len}{~n}
+{:else}
+    Else Exists $idx: {$idx}{~n}
+    Else Exists $len: {$len}{~n}
+    Else Exists {?$idx}$idx is true{:else}$idx is false{/$idx}{~n}
+    Else Exists {?$len}$len is true{:else}$len is false{/$len}{~n}
+{/things}
+
+{^things}
+    Inside Not Exists $idx: {$idx}{~n}
+    Inside Not Exists $len: {$len}{~n}
+    Inside Not Exists {?$idx}$idx is true{:else}$idx is false{/$idx}{~n}
+    Inside Not Exists {?$len}$len is true{:else}$len is false{/$len}{~n}
+{:else}
+    Else Not Exists $idx: {$idx}{~n}
+    Else Not Exists $len: {$len}{~n}
+    Else Not Exists {?$idx}$idx is true{:else}$idx is false{/$idx}{~n}
+    Else Not Exists {?$len}$len is true{:else}$len is false{/$len}{~n}
+{/things}

js/test_cases/idx_and_len/number.json

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+{
+  "things": 7
+}

js/test_cases/idx_and_len/string.json

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+{
+  "things": "foobar"
+}