Fix #5517: Backlashes in output fail for escape sequences in Windows

[antrancs]

This is my very first PR. So if there's any suggestions or if I'm missing anything, please advise 😀

Summary
This is the fix for #5517 where backlashes in the output in Windows are broken if the directory path contains "\n". The stringifyLangArgs method has been modified so that "\n" will only be replaced by EOL if it is not preceded by a "\".

Test plan
The test project path is C:\Users\An\Documents\Projects\oss\name
Before

error An unexpected error occurred: "C:\\Users\\An\\Documents\\Projects\\oss\ 
ame\\package.json: Unexpected token U in JSON at position 139". 

After

error An unexpected error occurred: "C:\\Users\\An\\Documents\\Projects\\oss\\name\\package.json: Unexpected token U in JSON at position 139". 

Also added automated tests.

[BYK]

If you can add a test case for it, I think the PR is good to go :)

Thanks a lot!

[BYK]

Let's add tests for this.

Also a comment in the new function, explaining why and using descriptive variable names instead of $0 and $1 would be great.

[antrancs]

@BYK : Thanks for the review :D. I just updated the function and added tests for it.

[BYK]

Nice!