* Fix more Compiler Errors for TS 2.4 in the VSCode codebase

* Fix compile error for 2.3

- 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

From: https://github.com/Microsoft/TypeScript/issues/15352

TS 2.3.1 introduced a breaking change by [switching to covariant types for callbacks](https://github.com/Microsoft/TypeScript/wiki/Breaking-Changes#covariance-in-callback-parameters). This change tries to fix these compiler errors in the workbench codebase