Spec for Windows Terminal Process Model 2.0 (#7240)

### ⇒ [doc link](https://github.com/microsoft/terminal/blob/dev/migrie/s/5000/doc/specs/%235000%20-%20Process%20Model%202.0/%235000%20-%20Process%20Model%202.0.md) ⇐

## Summary of the Pull Request

This spec is _exceptionally long_, and is currently a work in progress. There are a few more things I'd like to have experimentally verified (though, I'm fairly certain they _will_ work, with the right combination of flags and such). Additionally, a few sections have remaining TODOs before the spec is finished. However, this spec is already fairly long, and I want to give people as much time to get their eyes on it as possible.

### Abstract

> 
> The Windows Terminal currently exists as a single process per window, with one
> connection per terminal pane (which could be an additional conpty process and
> associated client processes). This model has proven effective for the simple
> windowing we've done so far. However, in order to support scenarios like
> dragging tabs into other windows, or having one top-level window with different
> elevation levels within it, this single process model will not be sufficient.
> 
> This spec outlines changes to the Terminal process model in order to enable the
> following scenarios:
> 
> * Tab Tearoff/ Reattach ([#1256])
> * Run `wt` in the current window ([#4472])
> * Single Instance Mode ([#2227])
> * Quake Mode ([#653])
> * Mixed Elevation ([#1032] & [#632])


## PR Checklist
* [x] Specs: #5000
* [x] References: #1256, #4472, #2227, #653, #1032, #632, #492
* [x] I work here

## Detailed Description of the Pull Request / Additional comments
_\*<sup>\*</sup><sub>\*</sub> read the spec  <sub>\*</sub><sup>\*</sup>\*_

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