non-stream,non-await transform

```ts
for await (const { path } of expandsGlob(`examples/*/index.{html,svelte,astro,vue}`)) {
  rewriter.tranform(await Deno.readTextFile(path))
  console.debug(rewrite.peek())
}
const concatenated = rewrite.peek()
rewrite.flush() // reset buffer string
rewrite.peek() // empty string
```