The `keys()` function returns an array containing all keys of the given
dictionary value. The keys are sorted in declaration order.

-- Testcase --
{{ keys({ "foo": true, "bar": false, "qrx": 123 }) }}
-- End --

-- Expect stdout --
[ "foo", "bar", "qrx" ]
-- End --


If the given argument is not a dictionary, the function returns `null`.

-- Testcase --
{{ keys(true) === null }}
-- End --

-- Expect stdout --
true
-- End --