Async Parsing

Nobody has claimed this yet.

Right now, a single Parser can only parse in a synchronized format, and the Parser only has one Environment that is used during parsing. On the other hand, all Methods are immutable and would work perfectly fine in an asynchronous environment.

In most cases, JagTag is used just for text processing, and thus async processing is not needed (and would mean more overhead to create additional Environments). However, JagTag is perfectly viable for actions beyond text processing that may take more time (web requests, image manipulation), and forcing creation of a new Parser for each parse is unnecessary.

Allowing asynchronous parsing would mean re-using the Parser but doing one of the following:
* Supplying an Environment to be used during parsing
* Allowing an empty Environment to be created when parsing begins
* Utilizing some sort of 'Action' template that allows 'building' the Environment and then parsing using it

Start by inspecting the Parser, Environment, and Method APIs to understand how parsing state is created and shared. Clarify which asynchronous lifecycle is required and define completion criteria around safely reusing one Parser for concurrent parses without sharing mutable Environment state.

Written by the indexing model from the issue text.