* Expose more shared properties from TextDocument on CustomDocument

Copies some shared properties from `TextDocument` to `CustomDocument`. We may want to consider this since `CustomDocument` is intented to be subclassed, and there is the risk after shipping the custom editor api that one of these subclasses could implement a property/method such as `version` which would break if we added a `CustomDocument.version` property at a later time

For #77131

- Use a class for `CustomDocument` instead of an interface. Extensions can now add their own data to a `CustomDocument` by sublassing

- Renamed `resolveCustomDocument` to `openCustomDocument` and require that extensions return a `CustomDocument`

- Exposed edits on `CustomDocument`

- Made the third parameter of `registerCustomEditorProvider` a generic options bag that takes a `webviewOptions`

Test of #91129

Cleans up API and removes some unused features (e.g. paging)
Adds date formatting
Adds loading progress and message
Removes lots of console.logs 😁
Adds titles to diffs

Part of https://github.com/microsoft/vscode/issues/88391

Squashed commit of the following:

commit 2bbee1ef
Author: Pine Wu <octref@gmail.com>
Date:   Wed Jan 22 11:48:44 2020 +0100

    Separate testing changes

commit 047e81e9
Author: Pine Wu <octref@gmail.com>
Date:   Wed Jan 22 11:44:30 2020 +0100

    Tweak layout

commit 078b21f6
Author: Pine Wu <octref@gmail.com>
Date:   Tue Jan 21 15:20:07 2020 +0100

    UI changes

commit a869c0fa
Author: Pine Wu <octref@gmail.com>
Date:   Tue Jan 21 12:28:35 2020 +0100

    WIP

When the core references `vscode`, we only want to import the types and never generate a real import (which will fail to load). Use `import type` to better enforce this

This reverts commit 6c9b23ed, reversing
changes made to 085abd5f.

`appendMarkdown` is now the only way to use ThemeIcons