Add a .d.ts file…

[savetheclocktower]

…and some tests to ensure the .d.ts document everything in the Repository interface (and nothing that isn't there).

This one builds on #8 and thus will stay in draft until that PR lands.

I was looking for ways to add inline documentation for this module. Ordinarily I'd prefer to write JSDoc comments and then use tooling to generate a .d.ts from that… but git-utils defines some methods in JS and some methods in the native C++ bindings. Hence there's no place to hang a JSDoc comment for about half the methods.

Instead, I've hand-authored a .d.ts file. To ensure that the definitions are in sync with what actually exists in the repo, there's a new spec/api-surface-spec.js suite that inspects the .d.ts and the Repository class and ensures they agree.