Co-authored-by: Mike Griese <zadjii@gmail.com>
## Summary of the Pull Request
This spec is a subset of #5772, but specific to per-profile tab colors. We've had enough requests for that in the last few days that I want to pull that feature out into it's own spec, so we can get that approved and implemented in a future-proof way.
> This spec describes a way to specify tab colors in a profile in a way that will
> be forward compatible with theming the Terminal. This spec will be largely
> dedicated to the design of a single setting, but within the context of theming.
>
## PR Checklist
* [x] Specs: #1337
* [x] References: #5772
* [x] I work here
## Detailed Description of the Pull Request / Additional comments
_\*<sup>\*</sup><sub>\*</sub> read the spec <sub>\*</sub><sup>\*</sup>\*_
