Build: Copy Icons manifest file from Gutenberg

[mcsf]

Companion PR to Gutenberg's WordPress/gutenberg#74943

See the original PR for more context. This change will ensure that Gutenberg's packages/icons/src/manifest.php will be available in Core at wp-includes/icons/manifest.php along with the core SVG files at wp-includes/icons/library/*.svg.

Trac ticket:

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

Hi @mcsf! 👋

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