Match unused Ramp vendor credits to open bills

**Objective:** Find vendor credits that still have an unapplied balance, optionally for vendor id `{vendor-id}`, created in the last {months-back} months (default 12), and match them to open bills.

**Step 1: Credits**

Call `ramp-get-vendor-credit` with `created-at.from` set to the window start, and `vendor-id` if given. Keep credits whose remaining amount is above zero and which are not archived.

**Step 2: Applications**

For each credit kept, call `ramp-get-vendor-credit-applied-bill` with `bill-id` unset and match on the credit's reference number to see where it was already used.

**Step 3: Open bills**

For each vendor with remaining credit, call `ramp-get-bill` with `vendor-name` and list unpaid bills in the same currency that the credit could cover.

**Step 4: Report**

A table: vendor, credit reference, remaining amount, currency, age, candidate bills. Total remaining credit per currency.

**Rules & Guidelines:**

* Read only. There is no tool here that applies a credit, so recommend and let the user do it in Ramp.

**Validation:**

* If no credit has a remaining balance, say so.