[Reader] Scroll to the top if the tab bar button is tapped

[twstokes]

Fixes #17021

Description

This PR scrolls Reader to the top when the tab bar button is tapped.

Testing

1. In the Jetpack and WordPress apps, load Reader and scroll down some.
2. Tap the Reader tab bar button at the bottom, in the center.
3. Expect: Reader to scroll to the top.

Regression Notes

1. Potential unintended areas of impact

   • This only adds the functionality to ReaderStreamViewControllers. It's possible I missed another type that needs to scroll.

2. What I did to test those areas of impact (or what existing automated tests I relied on)

   • Manual testing above.

3. What automated tests I added (or what prevented me from doing so)

   • None.

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.

[wpmobilebot]

You can test the changes in Jetpack from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr19769-8d4f760 on your iPhone

If you need access to App Center, please ask a maintainer to add you.

[wpmobilebot]

You can test the changes in WordPress from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr19769-8d4f760 on your iPhone

If you need access to App Center, please ask a maintainer to add you.

[salimbraksa]

Both WordPress and Jetpack apps work as expected. LGTM! 🚀

[twstokes]

Squeezing this onto 21.4 since the release branch hasn't been cut yet. Auto-merge enabled.