Improve navigation positioning when clicking walkthrough highlights

[nikomatsakis]

Problem

When clicking on walkthrough highlights/comments, the navigation works but the target line appears towards the bottom of the buffer, making it harder to see the surrounding context and requiring manual scrolling.

Expected Behavior

When clicking on a walkthrough highlight/comment, the target line should be centered in the editor viewport or positioned at the top for better visibility and context.

Steps to Reproduce

1. Present a walkthrough with highlights/comments
2. Click on any highlight to navigate to the code location
3. Observe that the target line is positioned near the bottom of the editor viewport

Impact

This affects user experience when reviewing code walkthroughs, as users need to manually scroll to see proper context around the highlighted code.