Put sourcemap comment on new line at EOF

According to the [spec](https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k/edit#heading=h.lmz475t4mvbx) the source mapping comment should be placed on a new line at the end of the file. I've had problems with some sourcemaps not being picked up unless they were on their own line in the past.

I looked for tests to update for this but couldn't find any, would be good to have some eventually I guess!