Intent to prototype and ship: SelectedItem and SelectedItemText system colors.

[Emilio Cobos Álvarez]

Summary: Provide colors to differentiate a selected row from text
selection, fixing "Highlight" to return selection colors like other
browsers do.

Specification:
https://drafts.csswg.org/css-color-4/#valdef-system-color-selecteditem

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1693222

Preference: None

DevTools bug: N/A

Other browsers: WebKit and Blink don't implement these, but expose the
color via -webkit-focus-ring-color /
-apple-system-selected-content-background / ..., see the CSSWG
discussion linked in the bug for details.

web-platform-tests: Added as part of the work in the bug.

-- Emilio