Fixes #117489

With the new markdown renders, we are relying more on the back layer webview. This means that we need to do things like make sure the entire markdown cell inside the webview is draggable.

Currently this is not possible since the webview has a large left margin. This change makes the webview span the entire notebook editor and instead moves the margin into the individual norebook cells

We should only use the readonly label when an editable custom editor is on a readonly resource

No need to register the client as it's disposed via _disposePtyHost

Fixes #117012

fixes #118037

Closes #114979

Fixes #117628

Closes #117540

fixes #117975

make sure local dispoables are disposed and cleared, https://github.com/microsoft/vscode/issues/117675