Bulk importer for Fava

[umangv]

This is my first pass at making a bulk importer for Fava.

I find that one of the slowest parts of importing multiple transactions is that there's a lot of visual jumping. You have to look at the payee/narration, then jump to the postings, see if those make sense, sometimes you may need other hints (like the date or the amounts) before you're sure. Once you click next, you start over.

By grouping transactions by their "target account" (i.e., the "interesting" posting, where the "uninteresting" posting is the one that involves the account you are importing into), it's much easier to visually scan through the importer, catch problems, and make changes.

Known issues that I will address before converting from "draft" to "ready":

• lint/test issues

Known issues that I hope to address (either in this PR or in a follow up):

• Check boxes don't honor shift-clicks to select a range
• Doesn't use Svelte v5 runes. I tried migrating the current code to Svelte runes but it got nasty because the imports grouped by target account objects had references to items in the original entries array and the runes magic broke those references. This might be fixable with $state.raw() but I haven't experimented.

Nice to haves that I also hope to work on in the future:

• A drag and drop interface for moving transactions (either individually, or all selected transactions) to different sections.