[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>