Backport Add Overlay template part area

[getdave]

Backport of https://github.com/WordPress/gutenberg/pull/73359. The work in Gutenberg is currently experimental and so this PR is not yet ready to be merged into Core. If the Gutenberg feature proceeds then we will need to merge (an updated version of) this PR

Trac ticket: https://core.trac.wordpress.org/ticket/64334#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[bot]]

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[scruffian]

I think we can close this in favour of https://github.com/WordPress/wordpress-develop/pull/10692/