feat(editor): color preview swatches in the gutter (#1341)

Lines with a color literal show a swatch of the (first) color in the
gutter: `#rgb` / `#rrggbb` with or without alpha, `rgb()` / `rgba()` /
`hsl()` / `hsla()`, and CSS named colors in property values of style
sheets. Outside style sheets and markup only quoted hex values and color
functions count, so `#123` issue numbers and `#add` comments get none;
prose files get none at all.

Clicking a swatch in an editable file opens the system color picker; the
picked color replaces the literal in its own format (hex case and length,
the function's name and separators, the original alpha), as one undoable
edit per pick. Settings › Editor › General › Gutter Icons › Color preview
turns the swatches off.

Fixes #1206

src/platform/locales/text/zh.json

@@ -3983,6 +3983,7 @@
   "Implemented and overridden": "已实现和已重写",
   "Implementing and overriding": "实现和重写",
   "Implemented and overridden declarations": "已被实现和重写的声明",
+  "Color preview": "颜色预览",
   "Implementing and overriding methods": "实现和重写超类型方法的方法",
   "Replace all {count} occurrences": "替换全部 {count} 处",
   "↵ Apply · Esc Keep '{name}'": "↵ 应用 · Esc 保留 '{name}'",