Spec for unified keybindings and commands, and synthesized action nam…

…es (#6532)

## Summary of the Pull Request

This is another iteration on the Command Palette spec, from #5674. These were some ideas that were tossed around by @DHowett, @cinnamon-msft and myself, formalized here. I proposed this as an addendum to the original spec, since I think the first made sense atomically, and this only makes sense as a set of changes to the original. I didn't want to go hacking up the original doc to add this set of changes. 

**There are two proposals in this spec - they should be viewed as two atomic units. They can be accepted or rejected independently. I'm suggesting we approve both. They work _together_. I'm realizing now that this is worded confusingly, and it's on me to fix that.**

## PR Checklist
* [x] Another spec in the #2046 / #5400 saga
* [x] I work here
* [x] _is a doc_

> ## Abstract
> 
> This document is intended to serve as an addition to the [Command Palette Spec].
> While that spec is complete in it's own right, subsequent discussion revealed
> additional ways to improve the functionality and usability of the command
> palette. This document builds largely on the topics already introduced in the
> original spec, so readers should first familiarize themselves with that
> document.
> 
> One point of note from the original document was that the original specification
> was entirely too verbose when defining both keybindings and commands for
> actions. Consider, for instance, a user that wants to bind the action "duplicate
> the current pane". In that spec, they need to add both a keybinding and a
> command:
> 
> ```json
> {
>     "keybindings": [
>         { "keys": [ "ctrl+alt+t" ], "command": { "action": "splitPane", "split":"auto", "splitMode": "duplicate" } },
>     ],
>     "commands": [
>         { "name": "Duplicate Pane", "action": { "action": "splitPane", "split":"auto", "splitMode": "duplicate" }, "icon": null },
>     ]
> }
> ```
> 
> These two entries are practically the same, except for two key differentiators:
> * the keybinding has a `keys` property, indicating which key chord activates the
>   action.
> * The command has a `name` property, indicating what name to display for the
>   command in the Command Palette.
> 
> What if the user didn't have to duplicate this action? What if the user could
> just add this action once, in their `keybindings` or `commands`, and have it
> work both as a keybinding AND a command?
>

doc/specs/#2046 - Command Palette.md

@@ -1,7 +1,7 @@
 ---
 author: Mike Griese @zadjii-msft
 created on: 2019-08-01
-last updated: 2020-06-10
+last updated: 2020-06-16
 issue id: 2046
 ---
 
@@ -522,6 +522,13 @@ default. These are largely the actions that are bound by default.
 ]
 ```
 
+## Addenda
+
+This spec also has a follow-up spec which introduces further changes upon this
+original draft. Please also refer to:
+
+* June 2020: Unified keybindings and commands, and synthesized action names.
+
 ## Future considerations
 
 * Commands will provide an easy point for allowing an extension to add its