Refactor More Parsing Logic into Subparsers

[jnterry]

The main parser.ts file is getting very big and complicated.

We've split out parsing for footnote and cross reference environments into their own files - but its becoming clear that we are needing to duplicate code in these and the main parser file (eg, for generic text styling markersv (https://ubsicap.github.io/usfm/characters/index.html#special-text)

It would probably be possible to make the code more readable (and also remove duplication) by refactoring these "subparsers" into a resuable modular system, then each parser could be made up of multiple subparsers that can be combined depending on what symbols are allowed in a given environment.