Return encode result from `Image.encode` calls
Return encode result from `Image.encode` calls

Before this, there were one way to use the API:
call `await image.encode({ mozjpeg: {} })`
then use `encodedWith` by asserting that `mozjpeg` property exists on it

After adding return value to encode, people
will be able to use it like
`const { mozjpeg } = await image.encode({ mozjpeg: {} });`
which provides better type safety

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