Validate setting types at runtime

Because settings can be arbitrary JSON we should validate their type
when reading them in. TypeScript provides a bit of a false sense of
security here since at runtime they can be whatever the user wants, and
then the extension falls over in wierd and wonderful ways.

Add some basic type checking methods to make sure we're working with
booleans, constrained strings/strings, or arrays of strings.

Issue: #2007

Author: plemarquand