Set default RSS size GC trigger to 96 kiB

On Linux, we initiate a full GC any time we return from JS to C code and the RSS has grown above a certain trigger level. Previously, this trigger level was initially 0, meaning that a full GC would always occur while gnome-shell was starting up.

We now set the initial (and minimum) trigger to 96 kiB which is an arbitrary value that is not reached immediately during gnome-shell's startup, which I determined by testing.

This value seems quite conservative even for apps written in GJS, it's unlikely that they will grow out of control with a trigger of 96 kiB.