feat: add map-editor plugin with interactive OSM map and markers

Implement Phase 1 of custom Kirby plugin for editing interactive maps:
- OpenStreetMap base layer with MapLibre GL JS
- Click to add markers, drag to reposition
- Marker list sidebar with selection and deletion
- Auto-save with debounce (YAML format)
- Add marker button creates marker at current map center
- Max 50 markers per map (configurable)
- Clean UI with marker counter

Blueprint updated to use new map-editor field type instead of placeholder.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

public/site/plugins/map-editor/package.json

@@ -0,0 +1,17 @@
+{
+  "name": "map-editor",
+  "version": "1.0.0",
+  "description": "Interactive map editor plugin for Kirby CMS",
+  "type": "module",
+  "scripts": {
+    "dev": "npx -y kirbyup src/index.js --watch",
+    "build": "npx -y kirbyup src/index.js"
+  },
+  "dependencies": {
+    "maplibre-gl": "^3.6.0",
+    "js-yaml": "^4.1.0"
+  },
+  "devDependencies": {
+    "kirbyup": "^3.3.0"
+  }
+}