build: add type declarations to `/dist`

[luqven]

Before you submit:

• [x] Please read the contributing guidelines
• [x] Please search through the existing issues (both open AND closed) to see if your feature has already been discussed. Github issue search can be used for this: https://github.com/imgix/vue/issues?utf8=%E2%9C%93&q=is%3Aissue
• [x] Please take a moment to find out whether your idea fits with the scope and aims of the project

Is your feature request related to a problem? Please describe. Type declaration .d.ts file should be included for each bundle in /dist

Describe the solution you'd like Use a build step to place emitted declaration files into /dist

Describe alternatives you've considered Using the package.json types: "src/plugins/imgix-vue" field. This is sub-optimal because the types are not bundled together.

Additional context

• RFC https://github.com/imgix/vue/pull/516 showcasing using the package.json field