build: add type declarations to /dist

[luqven]

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• Please read the contributing guidelines
• 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
• 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 fix: expose types in package #516 showcasing using the package.json field

[luqven]

We could make use of vite-plugin-dts and add { rollupTypes: true }.

// [vite.config.js](https://github.com/imgix/vue/blob/9aebc6f129422ed21a9272f1a7e3fd9b3207794e/vite.config.js#L41)

import dts from 'vite-plugin-dts';
// ...
export default defineConfig({
  // ...
  plugins: [vue(), dts({ rollupTypes: true })],
  // generates imgix-vue.esm.d, imgix-vue.min.d
})

[luqven]

Assigned to @namack.

Don't hesitate to reach out if you need any help/feedback tackling this!