Use jsg::JsUint8Array for TextEncoder::encodeInto

jsg::BufferSource adds additional overhead and complexity
that is unnecessary for this specific API. Switching to
jsg::JsUint8Array yields a significant performance boost.

Author: jasnell

src/workerd/api/encoding.c++

@@ -486,10 +486,13 @@ jsg::BufferSource TextEncoder::encode(jsg::Lock& js, jsg::Optional<jsg::JsString
 }
 
 TextEncoder::EncodeIntoResult TextEncoder::encodeInto(
-    jsg::Lock& js, jsg::JsString input, jsg::BufferSource buffer) {
-  auto handle = buffer.getHandle(js);
-  JSG_REQUIRE(handle->IsUint8Array(), TypeError, "buffer must be a Uint8Array");
-  return encodeIntoImpl(js, input, buffer);
+    jsg::Lock& js, jsg::JsString input, jsg::JsUint8Array buffer) {
+  auto result = input.writeInto(
+      js, buffer.asArrayPtr<char>(), jsg::JsString::WriteFlags::REPLACE_INVALID_UTF8);
+  return TextEncoder::EncodeIntoResult{
+    .read = static_cast<int>(result.read),
+    .written = static_cast<int>(result.written),
+  };
 }
 
 }  // namespace workerd::api

src/workerd/api/encoding.h

@@ -218,7 +218,7 @@ class TextEncoder final: public jsg::Object {
 
   jsg::BufferSource encode(jsg::Lock& js, jsg::Optional<jsg::JsString> input);
 
-  EncodeIntoResult encodeInto(jsg::Lock& js, jsg::JsString input, jsg::BufferSource buffer);
+  EncodeIntoResult encodeInto(jsg::Lock& js, jsg::JsString input, jsg::JsUint8Array buffer);
 
   // UTF-8 is the only encoding type supported by the WHATWG spec.
   kj::StringPtr getEncoding() {
@@ -239,6 +239,7 @@ class TextEncoder final: public jsg::Object {
     // `Uint8Array`. The spec defines that this function returns a `Uint8Array` too.
     JSG_TS_OVERRIDE({
       encode(input?: string): Uint8Array;
+      encodeInto(input: string, buffer: Uint8Array): TextEncoderEncodeIntoResult;
     });
   }
 };