A marker interface of the JexlContext that indicates the interpreter to put this context
in the JexlEngine thread local context instance during evaluation.
This allows user functions or methods to access the context during a call.
Note that the usual caveats wrt using thread local apply (caching/leaking references, etc.); in particular,
keeping a reference to such a context is to be considered with great care and caution.
It should also be noted that sharing such a context between threads should implicate synchronizing variable
accessing the implementation class.