Add a package/application convenience layer

Introduce a new module, called package, which includes convenience
API and a set of simple conventions for packaged applications.
The module takes care of setting up private search paths for JS
modules and private typelibs, and it optionally does gettext too.
system: add a wrapper around exec()

The main use case for this is replacing the current program with
a debugger, so that you can run "my-js-app --debug --arg" which
becomes the equivalent of
/usr/bin/gdb --args /usr/bin/gjs /usr/bin/my-js-app --arg

In the future we might have a JS debugger too, but for now having
a C debugger is an improvement.
Package: add gdb integration

Allow running the launcher program with --debug to replace the
running instance with a GDB.