Add function to create file links

[sclu1034]

This will require adding the relevant functions to GLib first:

g_file_make_symbolic_link_async
g_file_make_symbolic_link_finish
g_file_make_hard_link_async
g_file_make_hard_link_finish

The synchronous g_file_make_symbolic_link already exists, but GLib does not yet support creating hard links at all.

The Lua wrapper should be rather straight forward. The signature should be

-- @async
-- @tparam string target The target to point the link to.
-- @tparam[opt=true] boolean symbolic If `true`, create a symbolic link.
-- @treturn[opt] GLib.Error
function File:link(target, symbolic, cb)
end