Adopt it for untitled and text file models. Use it for indicating dirty count in explorer and document edited on macOS.

Part of #78168

Discovered as part of #81574

As part of the fix for #81574, I discovered that a large number of callers were already passing `undefined` for this value. This fix just marks the type as possibly undefined as well

web - we need to prevent unload when we detect a pending save or auto save because long running operations are not allowed

We seem to use the readonly property way of exposing these values more widely. Standardizing this a bit across the codebase

#76442

This reverts commit 0e02825e.