Adds IMDB and Letterboxd icons to the Social Icon Block

[dhruvikpatel18]

Closes: #44789

What?

This PR adds missing IMDB and Letterboxd icons to the Social Icon Block in Gutenberg.

Why?

The Social Icon Block currently does not support IMDB and Letterboxd, making it difficult for users to link to their profiles on these movie-related platforms. Adding these icons improves the block’s functionality and consistency with other supported social networks.

How?

• Implemented the IMDB icon (imdb.js) using an optimized SVG.
• Implemented the Letterboxd icon (letterboxd.js) following the existing icon structure.
• Registered both icons within the @wordpress/icons package.
• Updated the Social Link Block to include these services.

Testing Instructions

1. Open the editor and create a new post/page
2. Add a Social Icons block
3. Click the "+" button to add a new social link
4. Search for "IMDB" and "Letterboxd" in the icon picker
5. Verify the icon appears and matches the design of other social icons
6. Add your Social profile URLs
7. Verify the link works correctly in both editor and preview

Screenshots or screencast

IMDB.and.Letterboxd.Screencast.mov

[Mamaduka]

Related #39609 (comment).

[github-actions]

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Unlinked contributors: jlavoie13, janmckell.

Co-authored-by: dhruvikpatel18 <[email protected]>
Co-authored-by: Mamaduka <[email protected]>
Co-authored-by: kathrynwp <[email protected]>

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[Mamaduka]

Closing in favor of #70261. The extensibility feature will ship in WP 6.9 and is already available in Gutenberg 21.1.