This PR adds a hledger module based on hledger-mode, separate from the currently existing ledger module.

I first tried to extend the ledger module with optional hledger support, but it was effectively becoming two modules in one and they could each exist independently so it seems like separating them is a cleaner solution.

Despite being very similar they're still different tools with different behaviours, even the website for hledger only describes it as "comparable" to ledger rather than being a different implementation of the same tool.

• I searched the issue tracker and this hasn't been PRed before.
• My changes are not on the do-not-PR list for this project.
• My commits conform to Doom's git conventions.