Text files: do not report dirty for readonly models

[bpasero]

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.

[bpasero]

Fixed via d108195

Verification:

• install LiveShare
• start a readonly collab session
• join this session from another insiders run (via --user-data-dir <some other dir>)
• type in the host
• verify the guest sees no dirty indicator