Add structure tag parsing

[Rocketknight1]

Extremely draft PR for now, mostly for discussion! This will definitely be changed and added to a lot before it's merged.

The idea here is that we extend the base structure tag classes with an optional ParserTag field. This lets us annotate blocks of text, JSON, or regex groups with ID tags. The parser schema can then use something like simple jmespath expressions to collect these, returning parsed output.

TODO:

• Add parsing metadata to structure tag classes (drafted, might need to modify it or add it to a couple of others)
• Figure out if we want a separate output structure that loads values from the parser metadata, or if the parser metadata directly specifies output paths
• Write parser logic
• Tests tests tests