Many include statements use forward slashes, while others use backwards
slashes. This is inconsistent formatting. For this reason, I changed the
backward slashes to forward slashes since that is the standard.
## Summary of the Pull Request
Despite being specified as `noexcept`, `FillConsoleOutputCharacterA` emits an exception when a call to `ConvetToW` is made with an argument character which can't be converted. This PR fixes this throw, by wrapping `ConvertToW` in a try-catch_return.
## PR Checklist
* [x] Closes#4258
* [x] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA
* [x] Tests added/passed: thanks @miniksa
## Detailed Description of the Pull Request / Additional comments
Following the semantics of other `FillConsoleOutputCharacter*` the output param `cellsModified` is set to `0`. The try-catch_return is also what other functions of this family perform in case of errors.
## Validation Steps Performed
Original repro no longer crashes.
* Ensure rights check and increments pass before assigning an object to a handle (since deallocation of handles will automatically attempt to cleanup).
