Proposal: Add array indexOf()/lastIndexOf() functions to JSONata

[jhorbulyk]

Inspired by #441

Currently, it doesn't appear as though JSONata has simple functions that are equivalent to indexOf() or lastIndexOf() that exist in languages such as JavaScript. It probably makes sense to add them.

[andrew-coleman]

Perhaps something along the lines of https://www.w3.org/TR/xpath-functions-31/#func-index-of