Update existing line numbers earlier in transistion

[mhartington]

https://github.com/pomber/code-surfer/commit/f4529caa662123443e64acae2fd8660c22ee1b97 added line number back to code surfer (thanks for that!) but I noticed a small inconsistency with how the existing line numbers are updated.

When a new step is animated in, it will animate the incoming text, then update the line numbers after the animation has completed. When stepping back, the line numbers will update, then the text will animate out.

Would it be possible to change when the line numbers update so that they update before the animation happens? Currently it feels a bit jarring to have the line number update after the step has been animated in.