The `rollup-plugin-license` has been using too many deprecated dependencies, so it is necessary to remove it.
As an alternative, this changes uses Rollup `output.banner` to insert copyright information. Since `terser` runs after `output`, it is not possible to insert the Front Matter defining permlink for `sw.js` through the same way (Jekyll Front Matter is YAML rather than JS, which would cause errors with terser).
Therefore, _Jekyll Collection_ is now used to add permlink to `sw.js`, with the collection named `app`, and the directory placed in `_app`. This directory is not tracked by git, but it will be included when building the gem.
