Add ability to disable scrollAnimation

[vlsi]

Use case: there's a code sample that fits on a single screen. Then, there are steps that highlight the relevant lines/columns.

Actual behavior: as the lines are highlighted, CodeSurfer scrolls the code, which makes it harder to follow the code/highlights.

Expected behavior: there should be a way to avoid scroll animation (e.g. on a per-step basis), so the steps can highlight lines without code movement.

[vlsi]

As an alternative, it might be useful to decouple highlighting from the "focused lines".

That is by default CodeSurfer might focus on the highlighted lines, however, a user might specify which lines should be "focused". Then the user might have a stable text position no matter which lines are highlighted.