TypeScript definition fixes

[ElteHupkes]

I'm attempting to convert a project using Buefy to Vue3 using this fork, so far very successfully (thanks!). It's also using TypeScript, and I realized those definitions have probably not been touched. Here's a few basic ones, which at least make sure app.use(Buefy) isn't an error, and programmatic dialogs don't suggest onConfirm / onCancel anymore. I want to look at the other definitions for programmatic access later.