feat(plugins): brand document-indexer gutter markers with marker icons (#2196)

Indexed markers (Edges.markers) were always drawn with the navigation glyph,
so a plugin couldn't tell its marker kinds apart. A [[contributes.markerIcons]]
entry can now claim them by key prefix (`markers`), with an optional
`tooltip` shown above the targets. The host matches the longest prefix and
sends the declaration id with each marker; the editor resolves the icon and
the translated tooltip from the projected declaration. Manifest only, under
the existing API 1.3 gate of markerIcons.

Closes #2194

docs/plugins/indexing.md

@@ -31,6 +31,22 @@ Keys are the plugin's own strings and ranges are UTF-16. When a key has
 definitions in the same detected module, other modules' definitions are left
 out. Several targets open a chooser.
 
+A marker's tooltip lists its targets. With API 1.3, a `[[contributes.markerIcons]]`
+entry can brand markers by key prefix; the longest matching prefix wins, and
+other markers keep the navigation glyph:
+
+```toml
+[[contributes.markerIcons]]
+id = "usages"
+icon = "icons/usages.svg"
+iconDark = "icons/usages_dark.svg"
+markers = ["usages#"]
+tooltip = "Usages of this class"   # shown above the targets
+```
+
+A marker published as `Anchor { key: "usages#User".into(), range }` then shows
+that icon. See `plugins/README.md` for the limits.
+
 Initial discovery reuses the host's project index. The editor schedules
 cancellable batches of at most 16 files; edits and file changes invalidate the
 affected entries before they are indexed again. Reloading, disabling or a