[lldb/interpreter] Move the history subcommand to session (NFCI)

This patch moves the history subcommand from the command to session
command. I think it makes more sense to have it there because as the command
usage suggests, it should be used to manage custom LLDB commands.

However, history is essentially tied to a debugging session and holds
all the commands (not specifically custom ones).

This also makes it more discoverable by adding an alias for it (mimicking
the shell builtin).

Differential Revision: https://reviews.llvm.org/D84307

Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>