Reader Comments: Show separator for last leaf comments

[dvdchr]

Refs #17476

This PR implements separator logic for the comment thread. Additionally, the separator is always hidden for the last row (as per the design).

I couldn't find a better term for this 😅 , but a "last leaf" comment is the last descendant of a top-level comment. I've also added a note in the code, but here's an example:

- comment 1
  - comment 2
    - comment 3 --> this is the last leaf of comment 1.
- comment 4
  - comment 5
    - comment 6
  - comment 7 --> this is the last leaf of comment 4.
- comment 8

To Test

• Enable the newCommentThread feature flag.
• Go to Reader > any comment thread.
• 🔍 Verify that the cell separators are correctly shown for every "last leaf" comment.
• Rotate the device to landscape.
• 🔍 Verify that the cell separators are displayed correctly.

Regression Notes

1. Potential unintended areas of impact
   n/a. Feature is hidden behind a flag.

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   n/a. Feature is hidden behind a flag.

3. What automated tests I added (or what prevented me from doing so)
   n/a. Feature is hidden behind a flag.

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger an installable build for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[dvdchr]

Classic Xcode 13 formatting. 🙄

[ScoutHarris]

I noticed one visual oddity. On an iPad (in this case iPad Pro 9.7-inch iOS15):

• In portrait, view post comments.
• Rotate to landscape.
• Notice the separator "artifacts" on the left.

[ScoutHarris]

I noted one UI issue. But since this is a WIP I'll go ahead and approve to not hold you up.

[dvdchr]

I noted one UI issue.

Thank you! This should be fixed in 31b7f3f. I noticed that this happened only after rotating from portrait to landscape. It doesn't happen when opening the comment thread while already in landscape orientation 😅. Here's a screenshot after rotating from portrait to landscape:

I've also tested to ensure that this does not cause any issues in iPhone. 🙂