Blogging Prompts: Move shared blogging prompt styles into an extension

[wargcm]

Ref #18402

Description

Moves some styling out of the dashboard prompt cell into a WPStyleGuide extension. These will be used by another view with similar UI.

Testing

To test:

• Enable blogging prompts feature flag
• Navigate to my site tab
• Verify blogging prompts card displays the same as previously

Regression Notes

1. Potential unintended areas of impact
   N/A

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

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

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 pr18453-bd05911 on your iPhone

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[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 pr18453-bd05911 on your iPhone

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

[ScoutHarris]

LGTM!

[ScoutHarris]

Just an FYI. The style names can get long, so sometimes we typealias them to something shorter, for example private typealias Style = WPStyleGuide.BloggingPrompts. You can do so if you like, either way is fine, just letting you know.

[wargcm]

For now, I think it's still alright since there's no nesting under the BloggingPrompts struct. If we add additional objects or some of the names get a little long, then we could probably do that.