Rename a few Dart files from camelCase to snake_case

[nilsreichardt]

According to the Dart Effective Guidelines, we should use snake_case for Dart file. DO name packages, directories, and source files using lowercase_with_underscores.

Used the following command to check if there are still files with camelCase:

find . -name "*.dart" | grep -E '[[:upper:]]' | awk -F/ '{print $NF}'

Is helpful for #493 so that labels are assigned correctly.

[github-actions]

Visit the preview URL for this PR (updated for commit e81ac3d):

https://sharezone-test--pr494-make-dart-files-snak-t19xz0rx.web.app

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