Improved interpretor path support

[Descolada]

Although I am aware of your policy of not accepting pull requests, I am leaving this here as a base for a potential fix for issue #303 whenever you get time to deal with this. Modify it as you wish.

This PR adds existsSyncEx and commandExistsSync functions to support accepting a command, file, or symlink as runtime paths. This fixes the issue where existsSync returns "false" in the case of a symlink pointing to the WindowsApps restricted folder.

1. commandExistsSync requires the package command-exists, this has been added to package.json
2. existsSyncEx uses lstatSync to check whether a file or symlink exists by checking for the ENOENT error. lstatSync is used instead of statSync because the latter fails with a "permission denied" error in cases of a symlink pointing to a restricted permission file.

For example, if AHK Microsoft Store edition is installed, commandExistsSync permits AutoHotkey command to be used instead of a full path, and existsSyncEx permits the symlink C:/Users/CURRENTUSER/AppData/Local/Microsoft/WindowsApps/AutoHotkey.exe which previously failed.

[zero-plusplus]

Thanks so much for letting me know that there is a problem with the path validation.

I will have to think about reducing npm package dependencies in the future so I can come up with a better solution.

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I will close this pull request in the future when this issue is resolved.