Plugin: Include Icons assets in ZIP

[mcsf]

What?

Follow-up of #72215, #71878, #74943

Include in the Gutenberg plugin ZIP file the assets (SVG and manifest.php) of the core library of icons.

Why?

Without these assets, the new Icon block (#71227) won't show any available icons for users of the Gutenberg plugin.

How?

Include the assets in the zip command invocation.

Why now?

This has gone unnoticed so far because, in Gutenberg, all testing happens by building the project from scratch and running the tests in that environment. Meanwhile, in Core, there's a dedicated Gutenberg-to-Core copy process that operates similarly, which explains why this was missed in the backport of the Icons API (Trac). Luckily, @desrosj is experimenting with WordPress/wordpress-develop#11019 and noticed the absence of icons there.

Testing Instructions

• npm run build:plugin-zip
• Install the generated gutenberg.zip file in a fresh WordPres site
• Either:
  • In a new post, add a Icon block, then select Choose icon to reveal the icon library in a modal
  • Ensure the icons are there
• Or:
  • Check the REST API: /wp/v2/icons

[ockham]

Reproduced the issue on a fresh WordPress install, using GB 22.6.0 RC2 ✅

Then tested the fix by using the asset produced by CI for the "Build Release Artifact" workflow on this PR ✅

LGTM 🚢

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Co-authored-by: mcsf <mcsf@git.wordpress.org>
Co-authored-by: ockham <bernhard-reiter@git.wordpress.org>

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[ockham]

I just cherry-picked this PR to the release/22.6 branch to get it included in the next release: 91c66f3