Add ability to import from openScale backup

[NewYearNewPhil]

Just stumbled upon trale, love the design!

I use openScale to pull measurements from a bluetooth scale, and I'd love to be able to take that data into trale.

openScale offers .csv backups (although this is somewhat hidden, the .csv files are only created using the "auto backup" feature, not the manual backup). I've attached an example for reference.

This PR adds support for this format to trale, differentiating the formats by their respective extensions (txt and csv). The openScale format parsing is naive, it doesnt reference the included headers but just targets the specific fields, so if this format changes, the logic here will likely break.

If there's interest, I can implement proper csv parsing logic, for now this approach works well for the latest openScale version.

Would love to see this get merged so I don't need to maintain my fork to use upstream updates! Let me know if any changes are needed or there are problems with this.