Prune: Keep x per day / week / month, no matter how many in total

## Have you checked borgbackup docs, FAQ, and open GitHub issues?

Yes

## Is this a BUG / ISSUE report or a QUESTION?

ISSUE

## System information. For client/server mode post info for both machines.

#### Your borg version (borg -V)

borg 1.2.8

#### Operating system (distribution) and version

Linux Mint 20

#### How much data is handled by borg?

389G

## Describe the problem you're observing.

My drive is now full, and I need to remove (prune) old backups.

However, I do not want to only keep the last 14 days, but two backups per week, no matter, how many that would be.
As far, as I understand the documentation, this is currently not possible.
Would it be possible to implement that?