Spec for the Suggestions UI (#14864)

## Summary of the Pull Request

> ## Abstract
> 
> Multiple related scenarios have come up where it would be beneficial
to display
> actionable UI to the user within the context of the active terminal
itself. This
> UI would be akin to the Intellisense UI in Visual Studio. It appears
right where
> the user is typing, and can help provide immediate content for the
user, based
> on some context. The "Suggestions UI" is this new ephemeral UI within
the
> Windows Terminal that can display different types of actions, from
different
> sources.
> 


## Detailed Description of the Pull Request / Additional comments

_\*<sup>\*</sup><sub>\*</sub> read the spec
<sub>\*</sub><sup>\*</sup>\*_

Similar to #14792, a lot of this code is written. This stuff isn't
checked in though, so I'm presenting formally before I start yeeting PRs
out there.

## PR Checklist
- [x] This is a spec for #1595. It also references:
  * #3121
  * #10436
  * #12927
  * #12863

Author: zadjii-msft

.github/actions/spelling/allow/allow.txt

@@ -95,6 +95,7 @@ shcha
 slnt
 Sos
 ssh
+sustainability
 stakeholders
 sxn
 timeline