LightBulb Preview buffer leaks via LSP

[KirillOsenkov]

When you open a LightBulb preview, DiagnosticsReactingDocumentoperationListener.DocumentOpened is getting called for the buffer that contains the preview text (even two because it's a diff?)

However I'm not seeing the DocumentClosed called for these buffers, which results in a pretty bad memory leak

[KirillOsenkov]

This is pretty impactful, we're leaking quite a lot on every lightbulb preview invocation.

We should also check other scenarios where we show buffers: Peek, diff, debugger textboxes, output window (?), etc

[dibarbet]

Had a quick chat with Kayle, we may need to set the content type back to inert to signal features to unhook from it.

[KirillOsenkov]

yup, that should do the trick