Refactor MetaMask extension to TypeScript

[brad-decker]

Background

It has been a long term goal of the MetaMask engineering team to convert all organization JavaScript code to TypeScript to take advantage of it's developer experience and type safety. Our goal was to convert all of the extension dependencies that we maintain to TypeScript first so that we could rely upon the types we create for them. We have progressed that goal a great deal, with only a handful of repositories remaining that have not yet been converted. Our new strategy is to try and convert leaf node files with few dependencies to use TypeScript, and then work our way to the trunk files. If we hit files that bring in dependencies that are not yet typed we will resolve to find types for them in the @types DefinitelyTyped repo. If those do not exist, or we own the dependency that is untyped, we will work around them until we have types.