Spec for shell integration marks (#14792)

## Summary of the Pull Request

> ## Abstract
> 
> _"Shell integration" refers to a broad category of ways by which a
commandline
> shell can drive richer integration with the terminal. This spec in
particular is
> most concerned with "marks" and other semantic markup of the buffer._
> 
> Marks are a new buffer-side feature that allow the commandline
application or
> user to add a bit of metadata to a range of text. This can be used for
marking a
> region of text as a prompt, marking a command as succeeded or failed,
quickly
> marking errors in the output. These marks can then be exposed to the
user as
> pips on the scrollbar, or as icons in the margins. Additionally, the
user can
> quickly scroll between different marks, to allow easy navigation
between
> important information in the buffer.
> 
> Marks in the Windows Terminal are a combination of functionality from
a variety
> of different terminal emulators. "Marks" attmepts to unify these
different, but
> related pieces of functionality.


## Detailed Description of the Pull Request / Additional comments

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

In all seriousness, I've already implemented a pile of this. This is
just putting the finishing touches of formalizing it.

## PR Checklist
- [x] This is a spec for #11000 and everything linked to that.