improve vscode notebook experience

[brettfo]

Some quick quality-of-life improvements for the VS Code notebook extension:

• Support DisplayedValueProduced and DisplayedValueUpdated events. This enables better output for #r nuget....
  • N.b., when adding a NuGet package, the kernel returns values with two separate valueId values, one is a GUID, the other is the package name. These values specify what display value to update. Because of this there are separate display events which causes both "Installed" and "Installing" messages to appear. This is a kernel/package restore issue and will have the correct behavior when the valueIds match.
• In addition to returning formatted values, also return the raw value with a magically-determined mime type. This enables stuff like native JSON output instead of falling back to a formatted string.
  • N.b., VS Code notebook outputs must have a mime type, so we have to provide something. The current magic simply assigns application/json if the returned value is a JSON object, and text/plain otherwise. This can be expanded in the future when necessary.
• Properly parse and display formatted values with the mime type application/json.
• (Bonus) Add local build/deploy instructions in README.md