fix: include `assets/search.svg` in npm release

[TonyRL]

Pull Request

What does this PR do?

Fixes asset/search.svg being excluded from npm release.

Steps to reproduce

1. Follow the official guide to add the plugin
2. Run yarn docs:build
3. Expect the following error messages

   (undefined) ./node_modules/vuepress-plugin-meilisearch/MeiliSearchBox.vue?vue&type=style&index=0&id=512bb40d&prod&lang=stylus& (./node_modules/mini-css-extract-plugin/dist/loader.js!./node_modules/css-loader/dist/cjs.js??ref--13-oneOf-1-1!./node_modules/vue-loader/lib/loaders/stylePostLoader.js!./node_modules/postcss-loader/src??ref--13-oneOf-1-2!./node_modules/stylus-loader??ref--13-oneOf-1-3!./node_modules/cache-loader/dist/cjs.js??ref--0-0!./node_modules/vue-loader/lib??vue-loader-options!./node_modules/vuepress-plugin-meilisearch/MeiliSearchBox.vue?vue&type=style&index=0&id=512bb40d&prod&lang=stylus&)
   Module build failed (from ./node_modules/mini-css-extract-plugin/dist/loader.js):
   ModuleNotFoundError: Module not found: Error: Can't resolve './assets/search.svg'

PR checklist

Please check if your PR fulfills the following requirements:

• [x] Does this PR fix an existing issue?
• [x] Have you read the contributing guidelines?
• [x] Have you made sure that the title is accurate and descriptive of the changes?

Thank you so much for contributing to Meilisearch!

[bidoubiwa]

My bad, I forgot to merge this

[bidoubiwa]

bors merge

[meili-bors[bot]]

Build succeeded:

• linter-check
• Tests (Node.js 12)
• Tests (Node.js 14)