Allow theming the diff view's diagonal fill with diffEditor.diagonalFill

Ensure @rematch and nextEmbedded can be used together in Monarch grammar

One was actually SystemMetaData, all others were either dropped or
replaced with sanitized versions classifiable as SystemMetaData.

Needs - sync language change to EH, and add status bar to markdown editor cells

Fixes #95615

When checking if we can persist a webview, we check to see if any reviver is registered for it. However this may activate extensions that register the reviver

For background webview that have been restored we can skip this check since we know they were persisted in the past

selectively propagate debugging arguments for payload

Focus terminal after dragging a file to terminal

Part of #90528