Fix the Global Styles being Applied Incorrectly in Search Block

[Vedant-Gandhi]

What?

This PR addresses the issue where global styles are applied properly to indiviual text and button.

Why?

Issue - #68195

How?

This PR adds the experimental selectors to select the button and input element.

Testing Instructions

1. Open the Site Editor using Appearance > Editor.
2. Click on Style > Blocks.
3. Select the Search Block and change the border color, width and radius.
4. Ensure the styles are being applied to the indiviual input and button element.
5. Now Create a new Post/Page and add the Search Block.
6. Modify the border properties from sidebar panel and ensure they are being applied to the same elements.

Screenshots or screencast

final_change.mp4

[github-actions]

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[github-actions]

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