[DO NOT MERGE] Provide a standalone way to run rollup for stats gathering

[romaricpascal]

Just a late idea for running a more recent version of rollup for gathering stats, waiting for us to move to rollup 3, taking a different route than #3109:

Instead of running during our build, it bundles a one off entry, dynamically generated, that imports the content of our package. rollup-plugin-visualizer still serves as a stat collection tool and we'd need to look in the JSON stats it creates to highlight what we want.

I'm imagining multiple entries pointing to each of the govuk-esm files to ensure that when imported they only load the modules we expect them to. Especially when importing parts of the exports (eg. Accordion from all.mjs), we could check that treeshaking works as intended.

This could be:

• a route to look into collecting stats with rollup-plugin-visualizer before we set it up on our build. Allowing us to put all the effort on it rather than a bit with size-limit now, and digging deeper once we've moved to rollup 3.
• a route to check treeshaking output (though it would only be valid for rollup's way of tree-shaking)

How it works

It uses an npm workspace to let us install a newer version of Rollup for our shenanigans. That means you'll need to run it with:

npm start --workspace stats

It then configures rollup with:

• @rollup/plugin-virtual to create a custom piece of code to be used as an entry.
• rollup-plugin-visualiser to gather the stats and output them.

Futher development Quick try

Idea would be to generate a list of [filePath, importName?] entries based on a listing of the following format, filing results in files properly namespaced ([componentOrFileName]-[import].json for example:

module.exports =  [
[ 'all.mjs', '*'], // export '*' from 'all.mjs',
['all.mjs', '{Accordion}'], // export {Accordion} from 'all.mjs'
[ 'components/accordion/accordion.mjs], // export 'components/accordion/accordion.mjs
].map(function () {
// Create rollup configuration from each entry
});

[romaricpascal]

Spike work is complete and perf work carries on in #3279