Fix: transform file extension for `.ts` and `.tsx` assets inside `block.json` files

[fabiankaegy]

Related Issue/RFC: #414

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• [x] I have read the CONTRIBUTING document.
• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have added tests to cover my change.
• [x] All new and existing tests passed.
• [x] I have added a changeset to my PR. See CONTRIBUTING document for instructions

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: b13498ef0d76eb39a43d8d8fa79c6c4544e29a3e

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

| Name | Type | | ------------ | ----- | | 10up-toolkit | Patch |

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