Add snackbar notices for page creation in Navigation block

[sidharthpandita1]

What?

Closes #72620

Adds snackbar notices to provide immediate feedback when users create a new page from the Navigation block.

Why?

User testing with 20 participants revealed that users are uncertain whether page creation was successful when using the Navigation block's page creator. Without visual confirmation, users don't know if:

• The action completed successfully
• They need to wait longer
• They should try again

This lack of feedback creates confusion and a poor user experience, especially for less technical users who may not realize the page was created.

How?

This PR adds snackbar notices to the LinkUIPageCreator component in page-creator.js:

• Success notice: Displays "Page created successfully." when the page is created
• Error notice: Displays "Failed to create page. Please try again." when creation fails
• Uses the existing @wordpress/notices store for consistency with other Gutenberg notifications
• Snackbar-style notices are non-intrusive and auto-dismiss after a few seconds

The implementation adds:

1. Import of noticesStore from @wordpress/notices
2. useDispatch hook to access createSuccessNotice and createErrorNotice
3. Success notice triggered after successful page creation
4. Error notice triggered in the catch block when creation fails

Testing Instructions

1. Open a post or page in the editor
2. Insert a Navigation block
3. Click on the Navigation block to open the block settings
4. Click the "+" button to add a new page link
5. In the dropdown, click "Create page"
6. Enter a page title (e.g., "Test Page")
7. Click "Create page" button
8. Expected: A green snackbar notice appears at the bottom saying "Page created successfully."
9. The notice should auto-dismiss after a few seconds
10. Try creating another page to verify the notice appears consistently

Test error scenario:

1. use browser dev tools to simulate network failure
2. Follow steps 1-7 above

Testing Instructions for Keyboard

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Screenshots or screencast

Before	After
No visual feedback after clicking "Create page" - users uncertain if action completed	snackbar appears at bottom: "Page created successfully." providing immediate confirmation

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Note: This change addresses UX feedback from unmoderated user testing where participants expressed confusion about whether the page creation action had completed successfully.

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Co-authored-by: sidharthpandita1 <sidharthpandita@git.wordpress.org>
Co-authored-by: scruffian <scruffian@git.wordpress.org>
Co-authored-by: getdave <get_dave@git.wordpress.org>

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[sidharthpandita1]

@t-hamano

[getdave]

Thank you for the PR.

Checking out your branch it looks like you may need to bring in some upstream changes to your fork because I'm missing the Create page option in the Nav block.

If you could update and then rebase your PR that would be great.

[sidharthpandita1]

Thanks — I’ve rebased on upstream/trunk and updated the branch. @getdave

[scruffian]

Thanks for working on this, looks good!