fix: compile themeable package to CommonJS so it is importable

[amanabiy]

Mirror the components repo: compile src-themeable to CommonJS and point the exports entry at the file via default, so both import and require resolve to the CJS build.

Also drop designTokensTemplateDir: chat components reuse the core design tokens and ship no template-tokens directory, so the build failed with ENOENT once the import succeeded. The option is optional in theming-build; the design-tokens output is still generated from the preset.

ticket: AWSUI-62329

Description

Related links, issue #, if available: n/a

How has this been tested?

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.16%. Comparing base (6ed3348) to head (35acef6).

❌ Your project check has failed because the head coverage (88.16%) is below the target coverage (90.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #181   +/-   ##
=======================================
  Coverage   88.16%   88.16%           
=======================================
  Files          25       25           
  Lines         355      355           
  Branches      100      101    +1     
=======================================
  Hits          313      313           
  Misses         42       42           

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