Widgets: Add TypeScript project config

[retrofox]

What?

• Add widgets/tsconfig.json, a standalone TypeScript project that covers the sources under widgets/<name>/.
• Document the optional widget package.json in the @wordpress/build README "Structure" section.

Part of #77616

Why?

widgets/ had no TypeScript project of its own, so widget .tsx files fell back to an inferred project: @wordpress/* imports, *.module.css imports, and JSX surfaced as errors in the editor even though the runtime code was correct. The dependency-resolution half of this gap was addressed in #78463, which gave each widget its own package.json; this adds the TypeScript half.

How?

• widgets/tsconfig.json extends tsconfig.base.json with jsx: "react-jsx" (automatic runtime, no React import needed), rootDir: ".", noEmit with composite: false, and types: [ "style-imports" ] for CSS module imports.
• It is intentionally not registered in the root tsconfig.json references: it is a standalone type-check project, not part of the incremental build graph.
• The README gains one line listing package.json as an optional npm dependencies manifest in the widget structure tree (per review feedback on Widgets: Declare dependencies in a per-widget package.json #78463).

Testing

1. npx tsc -p widgets/tsconfig.json completes with no errors.
2. npm run lint:tsconfig passes.
3. Open any widgets/<name>/*.tsx in the editor: @wordpress/* imports, *.module.css imports, and JSX resolve.

Before	After

[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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[github-actions]

Flaky tests detected in 355d0ae.
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/26238269638
📝 Reported issues:

• [Flaky Test] should allow user to add and remove multiple local font files #62862 in /test/e2e/specs/admin/font-library.spec.js
• [Flaky Test] should move focus back to the Publish panel toggle button when canceling #77721 in /test/e2e/specs/editor/various/publish-panel.spec.js

[simison]

Do we need to add widgets path to the list at tsconfig.json to ensure build checks widgets in CI as well?

[retrofox]

Do we need to add widgets path to the list at tsconfig.json to ensure build checks widgets in CI as well?

Good catch, you're right. Added widgets to the root tsconfig.json references, so the "Type checking" CI step (tsgo --build) now picks it up. Verified locally that the widgets project builds clean as a reference.

This follows the setup of the other non-package-type-check configs already in that list (storybook, test/e2e, test/performance, test/storybook-playwright), which are referenced explicitly so CI checks them. widgets/tsconfig.json is noEmit + composite: false like test/storybook-playwright, so it slots in directly.