Add test script and initial test suite

The test runner bundles TypeScript tests with esbuild and executes them using Node's built-in test runner. Tests cover
document downloader behavior, path utilities, rmapi bridge configuration, style refinement, and ZIP file handling. Mock
implementations for Obsidian plugin interfaces enable isolated unit testing.

package.json

@@ -5,7 +5,8 @@
   "main": "main.js",
   "scripts": {
     "build": "node esbuild.config.mjs",
-    "dev": "node esbuild.config.mjs --watch"
+    "dev": "node esbuild.config.mjs --watch",
+    "test": "node tests/run-tests.mjs"
   },
   "devDependencies": {
     "@types/node": "^20.0.0",