fix: Update Node path to compile with appium 2 and make him cross-platform

[Dor-bl]

List of changes

appium.js Windows node path used in the integration tests has changed on appium 2.
This PR addresses the the issue and makes sure it's generic for all users and compiles with appium 2.
In addition, I made a small refactor: Moved Appium.js path retrieval to helpers.paths class

in theory, it should also fix one or more dotnet CI tests:
https://github.com/appium/dotnet-client/pull/650/checks

Types of changes

What types of changes are you proposing/introducing to the .NET client?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• New feature (non-breaking change which adds value to the project)
• Breaking change (fix or feature that would cause existing functionality not to work as expected)

Documentation

• Have you proposed a file change/ PR with appium to update documentation?

This can be done by navigating to the documentation section on http://appium.io selecting the appropriate command/endpoint and clicking the 'Edit this doc' link to update the C# example

Integration tests

• Have you provided integration tests to pass against the beta version of appium? (for Bugfix or New feature)

Details

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