As a preparation for the next commit, we now have a reduced version of
LibShell which can be used with BuggieBox. That version will not include
LibSyntax, LibURL or LibCodeComprehension, which are all quite heavy for
BuggieBox to include in its own binary.
This change ensures that users can use LibShell easily understand this
library now, because we have an actual library directory.
In addition to that, we move the test scripts to Tests/LibShell, to
match the usual pattern of putting test-related files in the Tests/
directory.
