Added virtual scrolling to diffs [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Adds virtual scrolling to the diffs app. This is in an effort to improve our diffs performance. With this change I am seeing (locally) improvements in TBT from 20+s down to 4+s.

For now this is behind a feature flag diffs_virtual_scrolling so that we can test it out and make sure it is working as expected.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #326773 (closed)