add wesnoth.show_lua_console, as a debugging aide

This allows some sort of "drop into debug" mode for lua if your
script detects an error.

We could consider to actually add an "option to attach debugger"
as an error handling mechanism, but it might be complicated
because of wesnoth exceptions which must go through lua and be
treated as lua errors, and we cannot handle two exceptions at once
obviously, so the lua interpreter is probably not safe to run that
way in general. Anyways this commit does not do that.
Merge branch 'add_lua_interpreter_launch_command'