start with a step back and use IWorkspace where needed, nuke workspace context service from ext host, #28526

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

* Use the extension descriptions available in the UI thread
* Don't wait for handling extension points until extension host is running
* Create the extension host process only after extension points have been handled
* Send the initial configuration as part of the initData to the extension host

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 #5649: Allow extension activation only after the main process acknowledges its receival of the ext descriptions