Add a function to convert WCS-based DS9 regions to supported coordinate frames

`Region` objects represent abstract shapes that are not tied to any particular image or WCS transformation. Therefore, WCS-based DS9 coordinate frames are not supported, e.g., `physical`, `detector`, `wcs[0;a-z]`, etc.

As a workaround, one can load such region files into DS9 with their corresponding image/WCS and then save the region with a coordinate frame supported by this package (e.g., image, ICRS, FK5, etc.). We could explore adding a function that does this conversion in the `regions` package.

ref: #403

Start by reviewing the Region abstraction and the existing DS9 coordinate-frame handling, then determine how an image and its WCS would be supplied for conversion. Define which WCS-based frames should be supported and how converted regions can be represented in the package; the issue does not identify specific files or tests.

Written by the indexing model from the issue text.