feat: implement banner plugin

- Add bannerPlugin for rendering banner experiences
- Creates fixed position banner at top or bottom
- Supports dismissable close button
- Auto-shows on experiences:evaluated event
- Emits experiences:shown and experiences:dismissed events
- Cleans up on sdk:destroy
- Comprehensive test coverage (24 tests)
- Add jsdom for DOM testing support

Closes #7

Author: prosdev

package.json

@@ -35,6 +35,7 @@
     "@tsconfig/node-lts": "^24.0.0",
     "@vitest/coverage-v8": "^4.0.16",
     "husky": "^9.1.7",
+    "jsdom": "^27.3.0",
     "tsup": "^8.5.1",
     "turbo": "^2.7.2",
     "typescript": "^5.9.3",