perf: add string flattening to reduce retained memory (node only)

add a memory footprint benchmark

benchmark/index.ts

@@ -2,12 +2,14 @@ import { output } from './output'
 import { tag } from './tag'
 import { demo } from './demo'
 import { layout } from './layout'
+import { memory } from './memory'
 
 async function main () {
   await output()
   await tag()
   await demo()
   await layout()
+  await memory()
 }
 
 main()