Specify typeroot (#4892)

This prevents tsc from picking up random types from parent directories
such as in situations like an element-web layered build, and generally
seems like good hygiene as we don't want to pick up random types from
whatever directory we happen to be checked out into.

tsconfig.json

@@ -10,7 +10,9 @@
         "declaration": true,
         "strict": true,
         "allowImportingTsExtensions": true,
-        "lib": ["es2024"]
+        "lib": ["es2024"],
+        "baseUrl": ".",
+        "typeRoots": ["./node_modules/@types"]
     },
     "include": ["./src/**/*.ts", "./spec/**/*.ts"]
 }