Update results display to use VS Code web view API

[MattIrv]

VS Code now exposes a web view API that we can use for the results grid, including an option to persist the web view when it's not active, which will fix issues like #916

I have some code for this ready to go in the feature/useWebviewApi branch, but it needs a bit more testing. Right now it opens results in a new web view every time you execute the query, even if a result tab for that query is already open. Also the "Started executing query at Line _" link doesn't work correctly if the results and query are in different editor columns.