Fix peer dependencies & deprecations

[WarningImHack3r]

The required Svelte dependency isn't correctly used: it should be installed as a dev and a peer dependency, not as a regular dependency. With the current state of your repo, it forces the user to install your exact version of Svelte, 3.38.2, alongside their own project's version of Svelte, leading to a duplicate. You can check prettier-plugin-svelte/package.json or svelte-eslint-parser/package.json as examples.

This PR fixes this, by enforcing a Svelte version selector that accepts basically anything inclusively above 3.38.2. Also, I replaced rollup-plugin-terser with @rollup/plugin-terser, because the one you're using is now deprecated.

All the changes have been tested by running successful npm i and npm run build.