Text files: do not report dirty for readonly models #89405
Description
So far, whenever the model of a text file changes, we would report it as dirty to the user. However, for readonly file systems, this is not ideal because neither could the user have changed the model nor can it save it.
Examples for readonly file system providers are our very own Git file system provider that a user should never see dirty files from. Other examples include LiveShare readonly sessions.
A change should be made such as readonly file models never report dirty when the model changes.
