Test extension samples

[jrieken]

• any os - @weinand

We have added a more complex sample to show how to use language features as command and how to use the text document content provider. Please go through the code and check that it works and explains itself: https://github.com/Microsoft/vscode-extension-samples/tree/master/contentprovider-sample

Also, the previewHtml sample got updated such that it now has a command-link: https://github.com/Microsoft/vscode-extension-samples/tree/master/previewhtml-sample

Last, make sure the sample repo builds and runs without hassle (npm i, F5 from with VSCOde etc)