Replace writeFile with appendFile for file output

[mayank-Pareek]

Description:

This pull request modifies the program to append data to the output file when multiple input files are provided and the --output flag is used. Previously, the program would overwrite the output file with each input file’s data. Now, it appends the results from each input to the specified output file.

Changes Made:

• Updated the logic to use fs.appendFile instead of fs.writeFile when multiple input files are passed along with the --output flag. This ensures that data from all input files is written sequentially to the same output file, rather than overwriting previous content.
• Added logic to output a warning message if the output file is not empty. File is checked using fs.readFile for empty check.

This fixes #12 , let me know if further changes are required.

[mayank-Pareek]

I have moved the file check logic out of loop hence, the output file will be checked for existent data only once. This change will overwrites any existing content before printing the first response.

I have not removed the warning message completely, however with this change it will be printed only once irrespective of number of input files. Let me know if you still want it to be removed completely.