Updates design of the diff expansion button

What does this MR do and why?

Updates the design of the diff expansion row to match the new designs. It moves the function definition into the expsnaion row whilst also moving the expand buttons into the gutter.

Screenshots or screen recordings

How to set up and validate locally

• Enable the updated_diff_expansion_buttons feature flag

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Closes #351801 (closed)

Related to #356703 (closed)