Separator: Surfacing the HTML markup control

[Infinite-Null]

What?

Closes #70441

Reverts the HTML element control in the Separator block back to the Advanced Inspector panel to restore consistent UI patterns and improve overall usability.

Why?

In PR #70185, the HTML element control was moved from the Advanced Inspector to the top-level Settings panel to improve accessibility. However, this change created a significant usability regression.

How?

This PR reverts the UI changes from #70185 by:

• Moving the HtmlElementControl back to InspectorAdvancedControls
• Keeping the helpful control itself (just moving its location)

Testing Instructions

1. Open the WordPress block editor (post or page)
2. Insert a Separator block
3. Open the block settings in the sidebar
4. Click on the "Advanced" section at the bottom of the inspector
5. Verify that the "HTML element" control is present and functional
6. Test changing between <hr> and <div> options
7. Confirm the separator renders with the correct HTML element in both editor and frontend

Screenshots or screencast

Before (Current)	After (This PR)

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[Infinite-Null]

Hi @t-hamano, I have reverted the changes and moved HTML element control to the Inspector advanced section. Please review it at your convenience.

[Infinite-Null]

While we could also use InspectorAdvancedControls to achieve the same result, I followed the approach used in the previous PR and implemented it using InspectorControls for consistency. Happy to adjust if there’s a preference for a different approach.

[t-hamano]

Either way is fine since they have the same meaning, but it appears that InspectorAdvancedControls exists for backward compatibility. Therefore, the current approach of using InspectorControls seems to be fine.

[t-hamano]

Thanks for the PR!

I support this PR, but I want to avoid moving the UI multiple times due to disagreements, as this creates cognitive load for users. Let's take a moment and wait for the additional feedback.

[t-hamano]

As discussion is still ongoing on #70441, let's pause moving forward with this PR.

After much thought, let's merge this PR for now. See #70441 (comment)