Indicate unsaved work with asterisk in terminal title

[adamjoer]

Fixes #522

This is a small change that ensures that files that have unsaved changes are shown in the terminal title with an asterisk. It's updated on each change and undo/redo action:

https://github.com/user-attachments/assets/c7f7edc2-2933-4f43-be4d-a14d99dc7f85

[adamjoer]

@microsoft-github-policy-service agree

[lhecker]

I made some adjustments to avoid the String copy. I also changed it to use U+25CF because that matches VS Code.