Add support for the exists tag.

js/test_cases/exists/README.md

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+Exists appears to follow the same truthiness rules that sections follow, rather than merely checking if a value exists.

js/test_cases/exists/input1.json

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

js/test_cases/exists/input10.json

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+{"things": {}}

js/test_cases/exists/input11.json

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+["cat", "dog"]

js/test_cases/exists/input2.json

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+{"things": {"name": "Alice", "keyboard": "K-Type"}}

js/test_cases/exists/input3.json

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+{"there_are_no_things": 4}

js/test_cases/exists/input4.json

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+{"things": "just a string"}

js/test_cases/exists/input5.json

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+{"things": false}

js/test_cases/exists/input6.json

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+{"things": null}

js/test_cases/exists/input7.json

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+{"things": 0}

js/test_cases/exists/input8.json

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+{"things": ""}

js/test_cases/exists/input9.json

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+{"things": []}

js/test_cases/exists/main.dust

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+{?things}
+Thing: {things}
+{:else}
+No things {.}
+{/things}