Promise support

[Jack-Barry]

I had completely forgotten about #73 until getting some notifications recently. I am no longer working on any Electron projects, but here's an initial pass at implementing promisified versions of has, get, and clear. Everything else could probably follow the same pattern, I just don't have a whole lot of extra time to dump into them for now.

• Needed to bump JSHint up to get async/await syntax
• I ran the tests on my machine on Node 16, before I noticed CI uses 12. Fingers crossed the fs.promises stuff works 🤞🏼

[Jack-Barry]

Closing this to clear up clutter in my open PRs list. Feel free to reopen if it ever gets reconsidered