Modularize operator detection and usage to allow for extensibility

[brettz9]

While the sandbox allows for custom filtering (even as custom functions), we might modularize the path parsing so that operator detection and usage can be externally extended.

[rattrayalex]

I'm interested in this, and we might be able to submit a PR. How would you like it to work / where can we get started?

EDIT: specifically, I'd like to be able to do things like this:

JSONPath({
  // find SchemaObjects anywhere in an OpenAPI spec 
  // that have an `enum` property and a `nullable` property set to true
  path: '$..@schemaObject()[?(@.enum && @.nullable === true)]' 
})

where I'd need to define @schemaObject() myself.

My impression is that this is currently possible with @other(), but then I only get one kind of custom type selector (and it's not very well-named…)

[brettz9]

Feel free to come up with whatever approach you like. I'm barely maintaining the project, so no firm commitments, but I can hopefully review any well-documented PR, or, proposal.