Add extension icon derived from the testium logo

Dark editor tile + the orange testium orbital mark + a </> code cue. SVG source
kept (excluded from the VSIX), 256x256 PNG referenced via package.json icon.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

package.json

@@ -5,6 +5,7 @@
   "version": "0.1.0",
   "publisher": "testium",
   "license": "EUPL-1.2",
+  "icon": "icon.png",
   "engines": {
     "vscode": "^1.80.0"
   },