perf(isFolderEmpty): store valid files in a set

[twlite]

This pull request includes changes to the is-folder-empty.ts file to improve performance and code readability. The most important changes include converting the validFiles array to a Set and updating the corresponding method to use the Set's has method.

Improvements to performance and readability:

• packages/create-next-app/helpers/is-folder-empty.ts: Converted validFiles from an array to a Set for improved lookup performance. [1] [2]
• packages/create-next-app/helpers/is-folder-empty.ts: Updated the isFolderEmpty function to use the Set's has method instead of includes.

[ijjk]

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