Simplify the cql2-text grammar (future version improvements?)

This is feedback from trying to implement cql2-text.
Implementers (or at least us) face struggles with the current grammar.

I think it comes down mainly to these two things:

1. Some of the capabilities from extension conformance classes are defined as separate rules. I think it would be much easier to simply define new possible values for operators or pre-defined functions identifiers (using the same grammar rule as function calls) for operators using a function call syntax (i.e., array/spatial/temporal operators and predicates). This would cut down the number of rules dramatically, and I think would also allow to make the requirements in each conformance class clearer.
2. Some rules seem to exist only to restrict the data types (e.g., numericExpression, characterExpression, temporalExpression...). However, this is purely a runtime concept, since the data type that a certain expression (e.g., a property) will evaluate to will depend on the queryables. Therefore I would not have used grammar rules (which are about the syntax) to make this distinction. Instead, I think what is needed for this is to have requirements and/or permissions that specify the interpretation if an unexpected data type is used in such a context.

I think simplifying these two aspects of the grammar would directly result in simpler parser implementations, greater ease of implementation and greater interoperability.

Start by reviewing the current cql2-text grammar and the extension conformance class rules described in the issue. Compare the separate operator, function, and data-type expression rules, then determine whether a future grammar revision can simplify them while making the conformance requirements clearer.