Use custom flag to identify regular transactions

[maread99]

Currently regular transactions are flagged '!' and tagged 'reg_txn'. The tag allows them to be identified. It might be preferable for a custom flag to do this job although the lexer for beancount v2 is hardcoded to only recognize flags defined in beancount.core.flags. If beancount v3 provides for custom flags consider using flags to identify expected transactions, for example, 'XR' for a regular expected transaction.

(EDIT Jun 24) HOWEVER, would need to maintain v2 compatibility until v3 becomes established. i.e. this is perhaps something to consider only when it'd be reasonable to drop support for v2.

[blaggacao]

regular transactions are flagged '!'

I think that is not actually working, maybe it is the reason why those transactions are still subject to duplicate detection and I had to implement https://github.com/maread99/beanahead/pull/16/files#diff-f6d54813d78a276ac3cd1b222af073eef6386d04bbade282405f4820b8f66126R65 ?