Need flag for enforcing pure XQuery 3.0

[Chris Hillery]

We need a runtime flag that a module may enable which will cause the compiler to throw errors/warnings if the module makes use of any features that are not pure XQuery 3.0.

We should probably use the enable/prohibit-feature mechanism to do this.

Not 100% clear yet whether we need corresponding flags for enabling/prohibiting any of our extensions (JSONiq, scripting..), or just this one to enforce clean XQuery 3.0.