<!-- Enter a brief description/summary of your PR here. What does it fix/what does it change/how was it tested (even manually, if necessary)? -->
## Summary of the Pull Request
Add checkbox for 'inherit from parent process' for starting directory
When checked, the textbox and browse button are disabled
If the starting directory is empty, the checkbox is automatically checked
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## PR Checklist
* [x] Closes#8761
* [x] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA
* [ ] Tests added/passed
* [ ] Documentation updated. If checked, please file a pull request on [our docs repo](https://github.com/MicrosoftDocs/terminal) and link it here: #xxx
* [ ] Schema updated.
* [x] I work here
<!-- Describe how you validated the behavior. Add automated tests wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed
<img width="328" alt="pardir1" src="https://user-images.githubusercontent.com/26824113/104529798-64038980-55bf-11eb-93fd-75e6cf1e2547.png">
<img width="317" alt="pardir2" src="https://user-images.githubusercontent.com/26824113/104529803-66fe7a00-55bf-11eb-89b6-5b35c8ab89b8.png">
9905bd5f09
