refactor(shell): combine lean and leanlua executables in a single executable

The main motivation is to allow users to configure/extend Lean using .lua files before loading the actual .lean files.
Example:
        ./lean extension1.lua extension2.lua file.lean

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(shell): combine lean and leanlua executables in a single executable