[#2550] Add touch and copyFile to Filesystem.

Problem
=======

We would like to have touch and copyFile method to compat filesystem.


Changes
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Implement copyFile.

Touch was already implemented but as private method only used in compat
tests.


How to test
==========

reviewers: @alibotean

check that changes make sense.

I suggest `destination_file_exists` for the former test and `destination_folder_exists` for the latter.