Feat: Add uniform image sizes toggle to Gallery Block

[yogeshbhutkar]

What, Why & How?

Adds support for maintaining uniform image sizes in gallery blocks, preventing the last image in a row from expanding to full width when there are fewer images than columns.

• Added new has-uniform-image-sizes class support.
• Used flex properties to maintain consistent image widths.
• Prevented image growth using flex: 0 0 width.
• Maintained existing gap calculations.
• Preserved default column behavior when uniform sizing is disabled.

Testing Instructions

1. Create a Gallery Block.
2. Add 4 Image Blocks to it.
3. Set the column count to 3.
4. Notice, by default the last image occupies the full width.
5. Try toggling the control labeled uniform image sizes and notice all the image blocks now take the same sizes.

Screencast

Closes: #68563

[github-actions]

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Co-authored-by: yogeshbhutkar <yogeshbhutkar@git.wordpress.org>
Co-authored-by: jartes <jartes@git.wordpress.org>

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[yogeshbhutkar]

The feature implementation is complete. I will update the test cases after the initial round of review.