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

plugins/README.md

@@ -634,6 +634,23 @@ several cross-file targets, publish document-index mappings; the existing
 chooser lists every matching definition. Command clicks are deferred; use a
 code action on the same line.
 
+Document-indexer markers (`Edges.markers`) use a declared icon when their key
+starts with one of its `markers` prefixes; the longest matching prefix wins,
+then declaration order. An optional `tooltip` (1–256 bytes, translated by the
+plugin's catalog) is shown above the marker's targets and needs `markers`:
+
+```toml
+[[contributes.markerIcons]]
+id = "usages"
+icon = "icons/usages.svg"
+markers = ["usages#"]          # up to 16 key prefixes of 1–256 bytes
+tooltip = "Usages of this class"
+```
+
+Keys without a matching prefix keep the navigation glyph. Older IDEs that do
+not recognize `markers` or `tooltip` reject the manifest at install; update the
+IDE to use them.
+
 Then `register_plugin!(MyPlugin, providers)` and implement
 `openidea_plugin::providers::Providers` for it: `completion`,
 `inline_completion`, `code_actions`, `definition`, `line_markers`. Each gets