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

src/features/plugins/markerStore.test.ts

@@ -43,7 +43,7 @@ it("invalidates both changed files and markers targeting them", () => {
         {
           path: "/p/A.xml",
           version: 1,
-          markers: [{ range, targets: [{ path: "/p/A.java", range }] }],
+          markers: [{ range, targets: [{ path: "/p/A.java", range }], pluginId: "p", icon: null }],
         },
       ],
     ]),