Add new GitHub workflow which checks file encodings

[stweil]

It checks whether files related to a push or pull request contain encodings which are not UTF-8.

The workflow was created with Claude.AI.

[stweil]

Examples: https://github.com/stweil/kitodo-production/actions/workflows/check_file_encodings.yml. See annotations for these actions, escpecially for the ones which failed.

[stweil]

A manual test which checks all files in the current directory structure can be done with similar bash code on Linux:

for file in $(find * -type f); do
  # Skip if file is binary or doesn't exist (was deleted)
  if [[ ! -f "$file" ]] || [[ -z "$(grep -Il '.' "$file")" ]]; then
    continue
  fi

  # Try to detect encoding using file command
  encoding=$(file -i "$file" | grep -oP "charset=\K.*")

  if [ "$encoding" != "utf-8" ] && [ "$encoding" != "us-ascii" ]; then
    echo "::error file=${file}::File is not UTF-8 encoded (detected: ${encoding})"
    EXIT_CODE=1
  else
    # Double-check with iconv
    if ! iconv -f utf-8 -t utf-8 "$file" > /dev/null 2>&1; then
      echo "::error file=${file}::File contains invalid UTF-8 sequences"
      EXIT_CODE=1
    fi
  fi
done

The test uses file to detect the trivial cases for which it also reports the wrong encoding.
If file does not find a wrong encoding, a full check with iconv then checks the whole file.