feat: add solution excercise m4-003 concatenate-multiple-files

[rosariospalla]

No description provided.

[aleattene]

I think that if you had many files, this solution would lead you towards the callbacks hell.
Try to solve it in another way, for example:

• using async-await (basic solution)
• or for a more challenging solution, considering you have a larger number of files... try managing it through Promise.all (advanced solution)