Implement saving regions to ECSV via astropy Table

I recently implemented IO to/from ECSV files via `astropy.table` in `specutils`, and coded up a quick minimal implementation to save out the spatial regions relevant to Jdaviz in https://github.com/spacetelescope/jdaviz/pull/2874 to provide export parity with spectral regions. I spoke to @larrybradley offline about upstreaming that code to here, and he pointed out that it would need to handle all the metadata available in `regions`, the other regions that we don't use in JDaviz, and implement a reader/parser as well. This issue is mostly to register my intent to do that at some point, and to provide a public place for any further discussion.

Start by reviewing the existing astropy.table ECSV implementation in specutils and the linked JDaviz PR #2874. Inventory the metadata supported by regions and the region types not used by JDaviz, then define reader and writer coverage with round-trip tests as the completion criteria.

Written by the indexing model from the issue text.