Core: Allow .mjs extension for CSF stories

[idesigncode]

Closes #23078

What I did

• lib/core-common
  • Added "mjs" to normalizeStoriesEntry default files glob pattern
  • Updated relevant inline snapshots
• lib/core-server
  • Updated the regex test for the csfIndexer to allow ESM .mjs type story files.
  • Added mock data for existing StoryIndexGenerator & stories-json tests
  • Updated relevant inline snapshots & expected test values
• lib/cli
  • Added "mjs" to the configureMain extensions
  • Updated relevant inline snapshots
• ui
  • Added "mjs" to blocksOnlyStories glob patterns
• Updated stories file extensions lists in docs

How to test

1. Run storybook with yarn start
2. Run yarn test
3. Ensure the following tests pass:

• lib/core-common/src/utils/__tests__/normalize-stories.test.ts
• lib/core-server/src/utils/StoryIndexGenerator.test.ts
• lib/core-server/src/utils/stories-json.test.ts
• lib/cli/src/automigrate/fixes/bare-mdx-stories-glob.test.ts
• lib/cli/src/generators/configure.test.ts

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Maintainers

• If this PR should be tested against many or all sandboxes,
  make sure to add the ci:merged or ci:daily GH label to it.
• Make sure this PR contains one of the labels below.

["cleanup", "BREAKING CHANGE", "feature request", "bug", "documentation", "maintenance", "dependencies", "other"]

[shilman]

Thanks for this @idesigncode ! I think you missed this one though

storybook/code/lib/core-common/src/utils/normalize-stories.ts

Line 11 in 2a12a93

const DEFAULT_FILES = '**/*.@(mdx|stories.@(tsx|ts|jsx|js))';