Have Shift+Tab remove whitespace within a line

Extracted from https://github.com/Microsoft/vscode/issues/32280#issuecomment-333843440 From @martinsuchan --- Shift+Tab does not outdent. Tested in *.ps1 files. No idea if this has worked in previous versions, just noticed it now. Edit it works when I'm on the start of the line, but not in between strings. In VS2017 it works even between strings. Not sure it this is native VS behavior or ReSharper, but I use it a lot.  - VSCode Version: Code 1.16.1 (27492b6bf3acb0775d82d2f87b25a93490673c6d, 2017-09-14T16:38:23.027Z) - OS Version: Windows_NT x64 10.0.15063 - Extensions: Kotlin|mat|1.3.0 csharp|ms-|1.12.1 PowerShell|ms-|1.4.3 java|red|0.11.0