Add a new function code:get_mode() can be used to detect how the code servers behaves

Rationale:
    Some applications (like erlide) have code to be loaded dynamically on a
    node. It may be slow to load everything upfront every time, so if the
    node is in interactive mode, we would like to just append to the load
    path. Currently, there is no direct way to detect if the node is running
    in embedded mode or not so that we can do the right thing.