Matching key-value lines to postings where applicable

[korrat]

Currently, the tree-sitter grammar recognizes all key-value lines within a transaction as children of that transaction node. This behaviour is in line with the beancount grammar and parser behaviour. However, it necessitates a post-processing step to match key-value lines to postings or the transaction itself.

With some light trickery in the grammar/scanner, it should be possible to match key-value lines to postings during parsing. However, this would cause some deviation of the grammar from beancount's.

Would this be an acceptable change in your opinion?