[SPIKE] Sass module migration

[colinrotherham]

Spike to migrate from @import to the Sass module system

Not for consideration, only for research into #5981 and comes in two parts:

• [SPIKE] Sass module migration #5982 (this PR)
• [SPIKE] Sass module @import backwards compatibility #5983

Optional imports restored

This PR mostly removes the workarounds necessary to resolve:

• Slow compilation of SCSS files in Rails apps #1671

As predicted previously in #1671 (comment)

Why Sass modules?

It lets the compiler find issues that previously needed tests

For example:

1. 01a4e9d Load missing members into all partials
2. afc5fb8 Typography partial does not extend %govuk-list placeholder from other partial
3. 103cb9a Private members suffixed with _ or $_ need renaming for public use in other partials
4. 0246d50 Stray call to mq() which breaks unless sass-mq has @forward
5. b2fb722 Workaround for $sass-mq-already-included does not work (but can be fixed)
6. Custom $govuk-font-url-function value does not work due to scope
7. Custom $govuk-image-url-function value does not work due to scope