Add strict to tsconfig.json

Author: wooorm

test.js

@@ -57,6 +57,7 @@ test('unist-util-source', (t) => {
     'generated'
   )
 
+  // @ts-expect-error: runtime.
   t.equal(source(null, file), null, 'missing')
 
   file = new VFile('a\r\nb')

tsconfig.json

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