For #94637

Allows the active paramter to be be specified per signature instead of for all signatures. If provided, this overrides `SignatureHelp.activeParamter`

For #77131

Fixes #93963
Fixes #94515
Fixes #94517
Fixes #94527
Fixes #94509
Fixes #94514
Fixes #93996
Fixes #93913

This removes explicit edits from the API and reshapes the API to more closely match VS Code's internal API. The change also tries to better express the lifecycle of backups

Fixes #93867
For #77131

This property is no longer needed (or very helpful) with the current api design

Changes over to use ts-expect-error: https://devblogs.microsoft.com/typescript/announcing-typescript-3-9-beta/#ts-expect-error-comments

This revealed a number of places where we no longer need to supress errors

`SemanticTokensBuilder`: Add `push` overload that accepts objects and support out-of-order calls (for #93614)

* 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