[REQ] Support for .sitebuilder archive

### Describe the solution you'd like
I have a file (attached in zipped form because GitHub) that is of a proprietary archive format (`.sitebuilder`), but functions like a `.7z` archive. The `7z` command-line utility can extract it without any extra configuration, but I've yet to find a GUI extractor that can handle it. It's super niche, but I work with these files from time to time and it'd be nice to have a way to handle them quickly on macOS.

File (zipped): [test.sitebuilder.zip](https://github.com/aonez/Keka/files/11482425/test.sitebuilder.zip)

### Additional context
There is no publicly-available documentation for this file format, but `7z` reports the file structure as a `zip` with an offset of `43`.