- Refactor configuration services to read settings from multiple folders
- Implement merging strategy for multi root workspace settings
- Implement a configuration model that can be reused across layers
- Implement getRoot API in workspace context service

- Create a simple workspace context service for standalone editor
- Use the test context service for tests

This reverts commit 25b4b172.

see https://basarat.gitbooks.io/typescript/content/docs/tips/nominalTyping.html and https://github.com/Microsoft/TypeScript/blob/7b48a182c05ea4dea81bab73ecbbe9e013a79e99/src/compiler/types.ts#L693-L698

fixes #5882