Add support for dimensionless spectral regions

Dimensionless unit SpectralRegions would seem to suffer from the same bug as #846. In conversations with @rosteen, there is a fair debate about whether SpectralRegions should support this usecase (considering... they're not spectral units). This issue is meant to encapsulate that discussion. Other interested parties I'm pinging: @eteq @pllim @camipacifici @orifox @PatrickOgle

The issue names no files, tests, or entry points. Start by reviewing issue #846 and the referenced discussion with @rosteen to understand the bug and the competing positions on dimensionless SpectralRegions. Done requires a resolved project decision about whether this use case is supported, followed by an agreed implementation scope and tests.

Written by the indexing model from the issue text.