Add forced-colors media feature

[meduzen]

From the very insightful MDN page about forced-colors:

The forced-colors CSS media feature is used to detect if the user agent has enabled a forced colors mode where it enforces a user-chosen limited color palette on the page. An example of a forced colors mode is Windows High Contrast mode.

Naming proposition:

• --forced-colors for forced-colors: active;
• --no-forced-colors for forced-colors: none.

This is different from the prefers-contrast feature, which for example responds to macOS “Increase contrast” setting and is already available in Double Dash.

Related resources:

• Windows High Contrast Mode, Forced Colors Mode And CSS Custom Properties
• CurrentColor SVG in forced colors modes

It’s currently only supported in Windows high contrast mode (Windows 10 provides 4 customizable themes: 2 dark, 2 light). However, it’s still good to consider improving UIs when @media (forced-colors: active):

• you can remove your theme switcher because nothing will visually change;
• you might need some tuning of some SVGs: for example, on block.sunappu.net, the border of the 2nd block (you have to tap on it 100 times to unlock it 😅) remains black, and becomes invisible because the high contrast background is now also black
  
• buttons color are now the same as text color (white) while links has now a very contrasted accentued color (yellow): it might be disturbing if buttons without border now looks like regular text.

These ideas have not been validated by any accessibility export.