Add shell_get_file_contents_utf8_sync(), use it instead of gio temporarily

Adding correct annotations to Gio.File.load_contents revealed that gjs
doesn't actually support array+length combinations.  For 3.0 this would
be invasive to fix, so add a method to ShellGlobal which does what
we need.

https://bugzilla.gnome.org/show_bug.cgi?id=646333