stream: update TextEncoderStream to align with the latest spec

This PR updated TextEncoderStream implementation to align with the latest WHATWG spec.

• transform(): https://encoding.spec.whatwg.org/#encode-and-enqueue-a-chunk
• flush: https://encoding.spec.whatwg.org/#encode-and-flush

Test cases of /test/fixtures/wpt/encoding/streams/encode-utf8.any.js (WPT for TextEncoderStream) are all passed now. (The previous implementation didn't support surrogate pairs.)