## Summary of the Pull Request
This is a spec specifically dedicated to Mark Mode. It's an addition to the Keyboard Selection spec. I felt that it makes the most sense to make this a separate PR because there's a lot of ideas that are very specific to Mark Mode, and this gives us the space to modify some of that behavior and get a good look at how other terminal emulators designed this feature.
## References
#2840 - Keyboard Selection Spec (base spec/branch/PR)
## PR Checklist
* [X] Contributes to #715
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