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/editor/core/cm/prefs.ts

@@ -432,6 +432,7 @@ export const GUTTER_ICON_GROUPS = [
   { id: "bookmarks", gutterClass: "cm-openidea-bookmarks" },
   { id: "inheritors", gutterClass: "cm-openidea-gutter-lineMarkers" },
   { id: "supers", gutterClass: "cm-openidea-gutter-lineMarkers" },
+  { id: "colors", gutterClass: "cm-openidea-gutter-colors" },
 ] as const;
 
 export interface PrefsOptions {

src/editor/core/cm/theme.ts

@@ -104,9 +104,10 @@ export const openideaTheme = EditorView.theme({
   "&.cm-openidea-methodSeparators .cm-openidea-methodSeparator": {
     boxShadow: "inset 0 1px 0 var(--border-method-separator, var(--border))",
   },
-  "&.cm-openidea-hideGutter-run .cm-openidea-gutter-run, &.cm-openidea-hideGutter-bookmarks .cm-openidea-bookmarks": {
-    display: "none !important",
-  },
+  "&.cm-openidea-hideGutter-run .cm-openidea-gutter-run, &.cm-openidea-hideGutter-bookmarks .cm-openidea-bookmarks, &.cm-openidea-hideGutter-colors .cm-openidea-gutter-colors":
+    {
+      display: "none !important",
+    },
   // Inheritance markers share one gutter: each group hides its own icons.
   "&.cm-openidea-hideGutter-inheritors :is(.cm-openidea-lineMarker-implemented, .cm-openidea-lineMarker-overridden), &.cm-openidea-hideGutter-supers :is(.cm-openidea-lineMarker-implementing, .cm-openidea-lineMarker-overriding)":
     { display: "none" },