Consuming components
Your package.json has a "svelte" field pointing to src/index.js, which allows Svelte apps to import the source code directly, if they are using a bundler plugin like rollup-plugin-svelte or svelte-loader (where resolve.mainFields in your webpack config includes "svelte"). This is recommended.
For everyone else, npm run build will bundle your component's source code into a plain JavaScript module (dist/index.mjs) and a UMD script (dist/index.js). This will happen automatically when you publish your component to npm, courtesy of the prepublishOnly hook in package.json.