Update FolderExplorerService.ts to add import @pnp/sp/files

[moudhafersalhi]

Without the import @pnp/sp/files the methid _getFiles will fail as web.getFolderByServerRelativePath(folderRelativeUrl).files will be undefined and therfore the next .select will fail.

adding the import fixed the issue

Q	A
Bug fix?	[ X]
New feature?	[ ]
New sample?	[ ]
Related issues?	fixes #X, partially #Y, mentioned in #Z

What's in this Pull Request?

Please describe the changes in this PR. Sample description or details around bugs which are being fixed.
When using the FolderExplorer with the option showFiles, the folderxplorer doesnt show the files + it shows the following error

Cannot read properties of undefined (reading 'select')

When investigating i found out that the import of pnp/sp/files was missing, which caused that the result of web.getFolderByServerRelativePath didnt contain a property files

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[github-actions]

Thank you for the first contribution to this project.

[AJIXuMuK]

Thank you, @moudhafersalhi for the fix!