Fix tree-shaking of inner modules for the Webpack integration

[romaricpascal]

We were not quite following the right step to optimise the build as it would be in an actual production set up while still retaining the possibility to inspect the content. This PR amends the Webpack build so tree-shaking is set up properly and internal utilities not used by single-component.mjs get excluded.

Tree-shaking in Webpack is not only a matter of setting usedExports (which still includes unused code, just doesn't export it), but also of having the TerserPlugin set up, as it's the one effectively removing the unused code.

Instead, tree-shaking is properly set up by using the production mode. We can then tweak the configuration of TerserPlugin, disabling mangling so we can look at class, function and variable names in the output.

Closes #4964

[github-actions]

📋 Stats

File sizes

File	Size
dist/govuk-frontend-development.min.css	113.32 KiB
dist/govuk-frontend-development.min.js	42.34 KiB
packages/govuk-frontend/dist/govuk/all.bundle.js	88.16 KiB
packages/govuk-frontend/dist/govuk/all.bundle.mjs	82.83 KiB
packages/govuk-frontend/dist/govuk/all.mjs	981 B
packages/govuk-frontend/dist/govuk/govuk-frontend-component.mjs	359 B
packages/govuk-frontend/dist/govuk/govuk-frontend.min.css	113.3 KiB
packages/govuk-frontend/dist/govuk/govuk-frontend.min.js	42.33 KiB
packages/govuk-frontend/dist/govuk/i18n.mjs	5.55 KiB
packages/govuk-frontend/dist/govuk/init.mjs	4.86 KiB

Modules

File	Size (bundled)	Size (minified)
all.mjs	78.45 KiB	40.31 KiB
accordion.mjs	22.71 KiB	12.85 KiB
button.mjs	5.98 KiB	2.69 KiB
character-count.mjs	22.4 KiB	9.92 KiB
checkboxes.mjs	5.83 KiB	2.83 KiB
error-summary.mjs	7.89 KiB	3.46 KiB
exit-this-page.mjs	17.1 KiB	9.26 KiB
header.mjs	4.46 KiB	2.6 KiB
notification-banner.mjs	6.26 KiB	2.62 KiB
password-input.mjs	15.15 KiB	7.25 KiB
radios.mjs	4.83 KiB	2.38 KiB
skip-link.mjs	4.39 KiB	2.18 KiB
tabs.mjs	10.13 KiB	6.11 KiB

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Action run for e3b3ca7

[romaricpascal]

note Unfortunately, Webpack does not have any option to merge a custom config over the default it uses when in production mode, so I've made sure to add back Webpack's default to be as close as possible to an actual build.

[romaricpascal]

note Splitting those on their own command. Despite GitHub using the -e flag for its default shell, which should fail fast, the grep for getFragmentFromUrl failing didn't lead to the failure of the check.