Add strict to tsconfig.json

Author: wooorm

test.js

@@ -5,6 +5,7 @@ import {toString} from './index.js'
 test('toString()', function (t) {
   t.throws(
     function () {
+      // @ts-expect-error: runtime.
       toString()
     },
     /undefined/,

tsconfig.json

@@ -10,6 +10,7 @@
     "declaration": true,
     "emitDeclarationOnly": true,
     "allowSyntheticDefaultImports": true,
-    "skipLibCheck": true
+    "skipLibCheck": true,
+    "strict": true
   }
 }