[lldb/Lua] Recognize "quit" as a way to exit the script interpreter.

Add a way to quit the interactive script interpreter from a shell tests.
Currently, the only way (that I know) to exit the interactive Lua
interpreter is to send a EOF with CTRL-D. I noticed that the embedded
Python script interpreter accepts quit (while the regular python
interpreter doesn't). I've added a special case to the Lua interpreter
to do the same.

Differential revision: https://reviews.llvm.org/D82272