JSON Filter Operators (Revised Attempt)

[Jermolene]

This PR is the culmination of work to design a basic set of filter operators for reading data from JSON strings.

• New filter operators to read arbitrary JSON structures #6522 was the first attempt. It was restricted to working on JSON strings that were stored in a tiddler, and couldn't operate on JSON strings from variables, or as partial results in filters
• Parameterised transclusions #6666 required the JSON operators, but couldn't use the implementation in New filter operators to read arbitrary JSON structures #6522 because of the restriction on JSON being stored in tiddlers, and so a new implementation was created that worked on JSON strings within filters.
• Polymorphic Filter Operators #6932 explores a performance optimisation to address the concern that the frequent conversion of data between JSON string and object form is inefficient
• The implementation in Parameterised transclusions #6666 was then updated to make the semantics compatible with Polymorphic Filter Operators #6932

The upshot is that this implementation of the JSON operators can be inherently somewhat inefficient, but that there is a viable plan to address the inefficiency in a later release.