Experiments: register gutenberg-dashboard-widgets flag

[retrofox]

Part of #77616

Context

This PR lands the experiment flag scaffolding for a modernized dashboard direction picking up on the design @fcoveram proposed in #74616, where the WordPress dashboard becomes a customizable, widget-based home screen. It also aligns with the admin-modernization thread in the 7.0 planning post, which lists "grid layout customization to dashboard experience or other screens" as an area to explore.

Gating everything behind a single experiment from day one keeps the plugin shippable without affecting core behavior while the API settles. Exports on the JS side will stay __experimental / private until the shape stabilizes, so iterating on the dashboard work imposes no compatibility obligations on the broader community.

Happy to share screenshots of the current prototype as soon as it's presentable.

What?

Registers a new gutenberg-dashboard-widgets entry in the Gutenberg experiments page and exposes window.__experimentalDashboardWidgets when the flag is enabled. No runtime code is gated yet — that wiring lands with the first runtime PR.

Why?

Future PRs will introduce a customizable dashboard widget framework (widget types, grid layout, core widgets). All of that runtime code needs to be gated behind a single experiment flag so it can ship in the plugin without affecting core behavior until it graduates.

This PR lands the flag scaffolding first so subsequent PRs can reference it from day one, following the same approach used by gutenberg-guidelines, gutenberg-form-blocks, gutenberg-media-editor, etc.

How?

Two minimal changes, modeled after neighboring experiments:

• lib/experiments-page.php — add a settings field to render the checkbox on the Experiments admin page.
• lib/experimental/editor-settings.php — inline a window.__experimentalDashboardWidgets = true script on wp-block-editor when the experiment is enabled.

No loader gate is added yet (lib/experimental/dashboard-widgets/ does not exist on trunk). That wiring will come with the first PR that introduces dashboard runtime code.

Testing

1. Check out the branch and rebuild: npm run build (no JS changes, but safe to run).
2. Visit Gutenberg → Experiments. Verify a new checkbox appears: "Dashboard: customizable widget home screen".
3. Leave it unchecked. In the block editor, confirm window.__experimentalDashboardWidgets is undefined.
4. Enable the checkbox and save. Reload the editor. Confirm window.__experimentalDashboardWidgets === true.
5. Disable the checkbox and save. Reload. Confirm the window flag is gone (undefined).

[github-actions]

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Co-authored-by: retrofox <retrofox@git.wordpress.org>
Co-authored-by: simison <simison@git.wordpress.org>

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[simison]

I'd probably just call it something like "New Dashboard Experience" for simplicity but anything really goes :-)

[github-actions]

Flaky tests detected in 8a0f348.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/24836143969
📝 Reported issues:

• [Flaky Test] post title and content are editable and blocks can be inserted #76524 in /test/e2e/specs/editor/various/post-content-focus-mode.spec.js