improved component composition and added tests.

src/lib/router.svelte.ts

@@ -56,7 +56,12 @@ function matchPattern(pattern: string, path: string): Record<string, string> | n
     if (prefixed) {
       const [, prefix, name] = prefixed
       if (prefix && !actual.startsWith(prefix)) return null
-      const value = decodeURIComponent(actual.slice(prefix.length))
+      let value: string
+      try {
+        value = decodeURIComponent(actual.slice(prefix.length))
+      } catch {
+        return null
+      }
       if (!value) return null
       params[name.slice(1)] = value
       continue