Webpack: Set extensionAlias mapping to support TypeScript's automatic extension substitution

[valentinpalkovic]

Closes #15633

What I did

I have added a default list of resolve.extensionAlias to the Webpack5 builder to support TypeScript configurations, where relative imports use file extensions to satisfy ESM module relative import rules. I have taken the same extension substitution, which TypeScript also applies internally when trying to resolve a file with an extension that doesn't exist.

// Button.stories.ts

// only Button.ts exists, but with `extensionAlias` set up, Webpack can resolve the path.
import { Button } from "./Button.js";

Checklist for Contributors

Testing

The changes in this PR are covered in the following automated tests:

• stories
• unit tests
• integration tests
• end-to-end tests

Manual testing

This section is mandatory for all contributions. If you believe no manual test is necessary, please state so explicitly. Thanks!

1. Run a Webpack5 TypeScript sandbox
2. Change the following settings in the tsconfig.json:

{
  ...
  "compilerOptions": {
    ...
    "moduleResolution": "NodeNext",
    "module": "NodeNext"
  }
}

3. Change the import in the story file to import the component via .js file extension. It has to be js instead of ts, because TypeScript doesn't transform import paths. TypeScript will instead substitute file extensions.

// Button.stories.ts

// only Button.ts exists, but with `extensionAlias` set up, Webpack can resolve the path.
import { Button } from "./Button.js";

Documentation

• Add or update documentation reflecting your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Checklist for Maintainers

• When this PR is ready for testing, make sure to add ci:normal, ci:merged or ci:daily GH label to it to run a specific set of sandboxes. The particular set of sandboxes can be found in code/lib/cli/src/sandbox-templates.ts

• Make sure this PR contains one of the labels below:

  Available labels

  • bug: Internal changes that fixes incorrect behavior.
  • maintenance: User-facing maintenance tasks.
  • dependencies: Upgrading (sometimes downgrading) dependencies.
  • build: Internal-facing build tooling & test updates. Will not show up in release changelog.
  • cleanup: Minor cleanup style change. Will not show up in release changelog.
  • documentation: Documentation only changes. Will not show up in release changelog.
  • feature request: Introducing a new feature.
  • BREAKING CHANGE: Changes that break compatibility in some way with current major version.
  • other: Changes that don't fit in the above categories.

🦋 Canary release

This PR does not have a canary release associated. You can request a canary release of this pull request by mentioning the @storybookjs/core team here.

core team members can create a canary release here or locally with gh workflow run --repo storybookjs/storybook canary-release-pr.yml --field pr=<PR_NUMBER>

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[yannbf]

Perhaps the declaration file extensions are not needed?

Suggested change

	'.js': ['.ts', '.tsx', '.d.ts', '.js', '.jsx'],
	'.mjs': ['.mts', '.d.mts', '.mjs'],
	'.cjs': ['.cts', '.d.cts', '.cjs'],
	'.js': ['.ts', '.tsx', '.js', '.jsx'],
	'.mjs': ['.mts', '.mjs'],
	'.cjs': ['.cts', '.cjs'],