Reset interpreter session and reuse the interpreter (isolation level)

**Describe the feature you'd like**
For each file, `byexample` spawns an interpreter to run the examples and it will shutdown after processing the file.

If multiple files have examples of the same language, `byexample` will still spawn and shutdown the interpreter for each file.

This is quite slow.

Some interpreters have the concept of `sessions`: independent execution sessions with a clean environment.

We could use it to reset the session instead of spawning another interpreter which it should be much faster.

This opens several *isolation* levels:
- process: restart each interpreter, always
- session: restart the session if supported, restart the interpreter otherwise
- cleanup: within the same session, perform some actions to clean up it.
- none: do not do anything, it is the user that must code her examples correctly. `byexample` must still do some clean up like resetting the clipboard (isolation == none is *not* a way to concatenate files)