Add Text Alignment support for Button Block

[Rishit30G]

What ?

Closes core/button for #60763

How ?

This PR adds the updated text alignment support in block.json for button block and moves away from the older textAlignment way which required code changes in edit.js as well

Testing Instructions

1. Add a new post
2. Insert a button block
3. Notice it has text alignment options

Screenshots or screencast

Screen.Recording.2025-05-22.at.10.29.00.AM.mov

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[t-hamano]

Thanks for the PR!

Unfortunately, we are currently unable to move forward with #60763 because text alignment support cannot be overridden on block instances. See the screenshot below:

.

As you can see, even though the block-level alignment should be centered, the global style default value (right) is respected.

To solve this, we need to move forward with #62260, but we haven't found an ideal approach yet.

For now, let's close this PR.