rhai: add new values() fn for headers with multiple values

[garypen]

fixes: #2255

We can't update the indexer until we release router 2.0, since that
would be an incompatible change, but we can add a new function which
provides access to multiple values.

Here's an illustrative snippet:

// Create some headers to work with:
response.headers["scalar"] = "value 1";
response.headers["array"] = ["value 1", "value 2", "value 3"];

// We still see scalar header keys as a single string
print(`scalar: ${response.headers["scalar"]}`);

// If we know we have an array use the `values` fn
print(`array: ${response.headers.values("array")}`);
print(`array[2]: ${response.headers.values("array")[2]}`);

// If we access a multi-value header using the indexer, then:
print(`array: ${response.headers["array"]}`);
// we get the existing behaviour, the first value as a string

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Needs documentation DONE