[SPIKE] Check behaviour of older Sass compilers with CSS custom properties

[romaricpascal]

This PR fixes how the hsl call is made (as older versions of Sass, including Dart Sass v1.0.0, confuse it with Sass' own hsl function).

It also adds an update to GitHub actions storing the compiled result for each Sass version we test, so we can peek at what's being generated.

For the purposes of using CSS Custom Properties, looks like everything is in order:

• custom properties starting with -- are present as expected in the output
• so are the var() calls

[github-actions]

📋 Stats

File sizes

File	Size
dist/govuk-frontend-development.min.css	112.04 KiB
dist/govuk-frontend-development.min.js	42.21 KiB
packages/govuk-frontend/dist/govuk/all.bundle.js	87.2 KiB
packages/govuk-frontend/dist/govuk/all.bundle.mjs	81.94 KiB
packages/govuk-frontend/dist/govuk/all.mjs	4.17 KiB
packages/govuk-frontend/dist/govuk/govuk-frontend-component.mjs	359 B
packages/govuk-frontend/dist/govuk/govuk-frontend.min.css	112.03 KiB
packages/govuk-frontend/dist/govuk/govuk-frontend.min.js	42.2 KiB
packages/govuk-frontend/dist/govuk/i18n.mjs	5.55 KiB

Modules

File	Size (bundled)	Size (minified)
all.mjs	77.67 KiB	40.18 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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