#11902 rewrite packages\ketcher-core\textFileTransformer.js from JavaScript to TypeScript

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How the feature works? / How did you fix the issue?

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Created textFileTransformer.ts with proper TypeScript typing:

• Imported Transformer from @jest/transform to type the transformer object
• Explicitly typed all parameters and return values
• Used the Transformer interface to properly type the entire object
• Added comprehensive JSDoc comments explaining the transformer's purpose
  Updated jest.config.js to reference the new TypeScript file (.ts instead of .js)

Removed the old textFileTransformer.js file

Verified - TypeScript compilation shows no errors ✓

Check list

• unit-tests written
• e2e-tests written
• documentation updated
• PR name follows the pattern #1234 – issue name
• branch name doesn't contain '#'
• PR is linked with the issue
• base branch (master or release/xx) is correct
• task status changed to "Code review"
• reviewers are notified about the pull request