Adds envelope icon

[crisbusquets]

What?

Add an icon for an envelope

Why?

Context: #65531

This icon is widely recognized and helps users quickly identify email-related functions, such as accessing and managing newsletters and interacting with mail interfaces. Its inclusion provides a clear visual representation of these types of communication, which is currently lacking in WordPress.

Screenshots

Testing Instructions

• Run Storybook locally npm run storybook:dev
• Open the icon library http://localhost:50240/?path=/story/icons-icon--library
• Search for the envelope icon

References

https://www.figma.com/design/e4tLacmlPuZV47l7901FEs/WordPress-Design-Library?node-id=100489-5216&node-type=frame&t=2IvEe1dEb69GvAMV-11

[github-actions]

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Co-authored-by: crisbusquets <cbusquets1989@git.wordpress.org>
Co-authored-by: jameskoster <jameskoster@git.wordpress.org>
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[github-actions]

👋 Thanks for your first Pull Request and for helping build the future of Gutenberg and WordPress, @crisbusquets! In case you missed it, we'd love to have you join us in our Slack community.

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[jameskoster]

It looks good to me :)

[enejb]

The changes look good. Thanks for the new icon @crisbusquets!

[mtias]

Thanks for the PR!