fix: import renderToPipeableStream in server.mjs for re-exporting

[3846masa]

Describe the bug

In Preact v10.22.1, renderToPipeableStream is re-exported from server.mjs.

However, the export { ... } from '...' ; syntax does not define a renderToPipeableStream variable, so the export default { renderToPipeableStream } part will cause an error.

This PR solves the above problem by explicitly importing renderToPipeableStream.

Code that reproduces the bug

https://gist.github.com/3846masa/bb3ade18b8a16d947f85920640ab35a2

[coveralls]

coverage: 99.612%. remained the same
when pulling b75edab on 3846masa-tmp:fix/export-render-to-pipeable-stream
into 212b1f6 on preactjs:main.

[rschristian]

Great catch, thanks!