Theme: Add Sass fallback helper root entrypoint

[mirka]

What?

Stacked on #79470

Adds a package-root Sass shim for @wordpress/theme/design-token-fallbacks and points the package export at that stable entry point.

Why?

The Sass fallback helper needs to work in both package-export-aware build tools and older Sass setups that resolve packages from node_modules with a load path. Exporting or documenting a generated src/prebuilt path would make package internals part of the public API.

This should unblock us to reinstall the changes that we reverted in #79429.

How?

Adds packages/theme/design-token-fallbacks.scss as a public shim that forwards to the generated helper. The package export now points to that shim with Sass/style/default conditions, and the docs use the same public @wordpress/theme/design-token-fallbacks import for both package importer and load-path Sass setups.

Testing Instructions

1. Run npm run storybook:build.
2. Confirm the build completes successfully.

[github-actions]

Size Change: 0 B

Total Size: 7.52 MB

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[github-actions]

Flaky tests detected in 152ba72.
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/28083481924
📝 Reported issues:

• [Flaky Test] should change image when user is changed #43593 in /test/e2e/specs/editor/blocks/avatar.spec.js

[github-actions]

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Co-authored-by: mirka <0mirka00@git.wordpress.org>
Co-authored-by: ciampo <mciampini@git.wordpress.org>
Co-authored-by: aduth <aduth@git.wordpress.org>

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[ciampo]

Nice, the new import path also feels cleaner 🚀

[ciampo]

TIL @forward

[aduth]

Are all of these needed? 🤔

[mirka]

I looked into this a bit more, and I've concluded that we want either:

1. sass condition only (supports both legacy and modern setups)
2. No conditionals at all (supports only the legacy setup, but matches the rest of the repo)

The sass condition seems to be the recommended way for the modern NodePackageImporter setup. So I guess it's good to start supporting that, too.