Forgot that missing environment variables are replaced with empty string.

Fixes #4895

- API to get a value given a key
- API to update value smartly
- Fine grained configuration change event
- Smart reload API
- Remove unnecessary APIs

This reverts commit f1d99584.

fixes #31906

- Provide folder specific keys

* 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

fixes #16064