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## Summary of the Pull Request
Adds directional modifiers for SplitState and convert those to the appropriate horizontal/vertical when splitting a pane.
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## References
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## PR Checklist
* [x] Closes#4340
* [x] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA
* [ ] Tests added/passed
* [x] Documentation updated. If checked, please file a pull request on [our docs repo](https://github.com/MicrosoftDocs/terminal) and link it here: #xxx
* [x] Schema updated.
* [ ] I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx
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## Detailed Description of the Pull Request / Additional comments
"vertical" and "horizontal" splits were removed from `defaults.json`, but code was added to parse those as `right` and `down` respectively. It is also the case that if a user has a custom hotkey for `split: vertical` it will override the default for `split: right`.
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## Validation Steps Performed
Split the pane using each of the new directional movements
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