Feature: Custom Working Directory (#1543)

* Added `customWorkingDirectory` and `useCustomWorkingDirectory` properties to `Scheme.Run`

* Use new `customWorkingDirectory` and `useCustomWorkingDirectory` when generating `XCScheme.LaunchAction`

* Updated ProjectSpec.md to document new `customWorkingDirectory` and `useCustomWorkingDirectory` properties

* Fix for not setting customWorkingDirectory in the toJSONValue function

* Added test to make sure usCustomWorkingDirectory value is true when the customWorkingDirectory is set to non nil

* Change to infer the value of SchemaGenerator.LaunchAction.useCustomWorkingDirectory based on the value of Schema.Run.customWorkingDirectory

* Removed useCustomWorkingDirectory from the project spec now that it is no longer user defined.