feat: lib: Allow special string %account in auto-posting rules (#1975)

[Xitian9]

This allows using the special string %account in auto posting rules. When run, this will be substituted with the account name of the matched posting.

This is a minimal implementation of #1975. More bells and whistles can be added if we decide what features exactly we want to support here.

We may want to use the string %(acct) or %acct instead of %account.

[simonmichael]

Thanks for this. I have seen the %(...) syntax somewhere, but do we use it anywhere in hledger ? %account seems better to me.

Is the example in the test correct ? 19% of 1190 should be 226.10 I though.

[Xitian9]

Happy for you to make the call on exactly which format to use. The advantage of the brackets is that strings like `%accountfoo` are hard to parse from a human perspective, while `%(account)foo` makes clear where the special string lies, bit this is clearly not a common use. My understanding of the example (taken from OP) is that you are selling an item costing 1000, and 19% VAT is applied, bringing the total amount of money you receive to 1190. However, you owe 190 of that to the government. You are therefore seeking to recover the original 1000, which means multiplying by 1/1.19, and the VAT is (1 - 1/1.19) times the original amount.

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[simonmichael]

Ah I see. I think for consistency I'd prefer to support just %account for now nevertheless. I think it will be rare for people to use %account immediately followed by letters.

[Xitian9]

Updated.

[simonmichael]

Thank you very much!