Enable undo-aware dirty tracking for TextDocument

[Third-Thing]

Goal: Enable application code to query whether a document has unsaved changes, so that developers can implement save-on-close and discard-confirmation without tracking dirty state themselves.

Strategy: Add is_dirty() and mark_pristine() to Document, implementing them in TextDocument utilizing the existing Buffer::is_pristine and Buffer::set_pristine.

fn is_dirty(&self) -> bool {
    !self.buffer.with_untracked(Buffer::is_pristine)
}

fn mark_pristine(&self) {
    self.buffer.update(Buffer::set_pristine);
    self.update_cache_rev();
}

Does this seem optimal?

I can make the changes.

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Status Update: Ready for review

[jrmoulton]

@panekj @dzhou121

[panekj]

no opinion, if it works for you and floem

[jrmoulton]

sounds good to me