I had initially seen some oddities around type augmentation for type definitions in subdirectories of `types`, and using Rollup seemed to help with that. However, now that all of Chart.js's main types are directly under `types`, there seems to be no need for this.
This simplifies the build process, since it no longer needs to use rollup-plugin-dts.
It also improves some third-party tools. For example, I'm in the habit of using WebStorm's "Go To Declaration or Usages" hotkey with third-party TypeScript definitions as a quick way of getting more information about an API. With the Rollup-generate types, that works poorly; WebStorm goes to the imported-and-re-exported symbol within the barely-readable machine-generated dist/chart.esm.d.ts file, and I have to navigate two more hops to find the actual definitions.
Explicitly target files that should be included in the npm package, making it 10x smaller by removing the docs, samples, scripts, sources, tests and other useless files.
In order to be compatible with any CSP, we need to prevent the automatic creation of the DOM 'style' element and offer our CSS as a separate file that can be manually loaded (`Chart.js` or `Chart.min.js`). Users can now opt-out the style injection using `Chart.platform.disableCSSInjection = true` (note that the style sheet is now injected on the first chart creation).
To prevent duplicating and maintaining the same CSS code at different places, move all these rules in `platform.dom.css` and write a minimal rollup plugin to inject that style as string in `platform.dom.js`. Additionally, this plugin extract the imported style in `./dist/Chart.js` and `./dist/Chart.min.js`.
Browserify isn't optimal bundling Chart.js because it adds too many internal wrappers, doesn't handle external/global dependencies and doesn't provide a way to generate ESM builds. Therefore, it seems the right choice to switch to rollup, so move all the build process in `rollup.config.js` and make Gulp to execute `rollup -c`.
We also had to switch to Terser instead of UglifyJS because this last one contains a breaking bug. Note that tests now use the exact same rollup config as our builds (the minified one) to ensure that the bundling and minification steps don't break anything. Finally, replace the `gulp watch` task by `gulp build --watch` to be consistent with the other `unittest` and `docs` watching syntax.
