feat: typed wrapper around localStorage

[chadoh]

In #29, the raw calls to localStorage made me uneasy for a couple reasons:

1. Lots of strings! If the name of one of those keys gets mistyped, bad things will happen. Better to have type-checking on the strings, or to store them in constants, or something.
2. No types on the storage

A tricky thing here is that we are implementing a starting point for other projects. We want to provide something minimal, yet powerful. I confess that what I've added here might not qualify as "minimal"!

I don't want these new projects to have unnecessary npm dependencies, so I re-implemented (a minimal version of) an existing library.

I think the storage.ts file is slim enough to be understandable, and has nice properties. If you define your keys like this:

Then when you import storage elsewhere, you get this typeahead on getItem:

And the return types for keys of different types are all well-typed: