Improve support for Windows High Contrast Mode and Internet Explorer 11

[lfdebrux]

This builds on work by @chris-gds for the accordion component refresh in #2257.

What

Add media feature -ms-high-contrast to media queries that change styles for Windows High Contrast Mode.

Add CSS property -ms-high-contrast-adjust where property forced-color-adjust is used.

For example, see draft PR #2290

Why

Internet Explorer 11 doesn't support the forced-colors media feature or the forced-color-adjust property, but it has equivalents in -ms-high-contrast and -ms-high-contrast-adjust.

We can port fixes for High Contrast Mode such as improved focus states for radios and checkboxes (#2264) that currently work in Edge / Chrome / Opera but not Internet Explorer.

According to the 2018 WebAIM survey of users with low vision, a higher percentage use Internet Explorer than Edge.

Who needs to know about this

Devs

Done when

• GOV.UK Frontend has support for Windows High Contrast Mode and Internet Explorer 11 Improve support for Internet Explorer 11 with Windows High Contrast Mode  #2290
• Documented how to add tweaks for Windows High Contrast Mode and other forced colour modes in future

[lfdebrux]

It would be nice if autoprefixer dealt with this for us, however it looks like this functionality won't be added: postcss/autoprefixer#1351

We could add a mixin for Windows High Contrast Mode (as suggested by @chris-gds), however even in that case we would need to be careful when using it to make sure we include prefixed CSS properties such as forced-color-adjust/-ms-high-contrast-adjust.

Also, there may be scenarios where we want to use the forced-color media query but not the -ms-high-contrast media query (thanks for flagging this @36degrees).