Resolve "Merge request refactor does not highlight selected line"

What does this MR do?

When user clicks linenumber, highlight that line

- Utilize Vuex state so all components on page are aware
- Only one line highlighted in entire MR changes
- Does not highlight empty line, i.e. only highlights right side in
parallel view if that line did not exist before.

What are the relevant issue numbers?

Closes #48496 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.