Fix generated TextEncoder#encode() return type

[mrbbot]

Hey! 👋 This PR fixes the auto-generated return type of TextEncoder#encode(). #1047 updated this function to return a jsg::BufferSource. This generates ArrayBuffer | ArrayBufferView as the TypeScript type, as jsg::BufferSource retains the input type when passed back out to JavaScript. TextEnoder::encode() uses BufferSource::tryAlloc() though which always gives a Uint8Array:

workerd/src/workerd/jsg/buffersource.c++

Line 89 in 2182afd

return BufferSource(js, v8::Uint8Array::New(buffer, 0, size).As<v8::Value>());

Therefore, an override has been added to force a Uint8Array return type as required by the spec.

Fixes #1149