Improve snackbar success notice UI with green accent and larger size.

[Ta5r]

What?

Part of #34306

• Improves the visual prominence of success notifications in the editor by:
• Increasing the snackbar size and padding
• Adding a green accent border for success messages

Why?

• Helps users more easily recognize successful post actions (like publishing or updating), enhancing clarity and feedback

How?

• Overrides .components-snackbar styles and adds .is-success border and padding rules, aligned with WordPress design tokens.

Testing Instructions

1. Open the block editor in WordPress.
2. Create a new post or page and enter some content.
3. Click Publish or Update.
4. Snackbar should appear with increased size and padding.
5. A green left border should appear, indicating success.
6. Snackbar should include clear text like “Post published.”
7. If a post link is included in the snackbar, a "View" button should appear.

Testing Instructions for Keyboard

1. Press Tab repeatedly to navigate through the block editor.
2. After publishing a post, continue tabbing:
3. Snackbar should receive focus if it's the next interactive item.

Screenshots or screencast

Before	After

[github-actions]

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[t-hamano]

Thanks for the PR!

Rather than simply submitting the approach proposed in the issue as a PR, let's first understand the issue and carefully discuss what is missing and what should be done. For example, the following points:

• It has been four years since the issue was submitted. Is this issue still valid?
• Do we need variations other than green, such as yellow, red, etc.?
• Ideally, wouldn't we need a status prop like the Notice component?