Add an interactive mode to the CLI

[alecthomas]

Introduce an interactive "prompt" mode for the CLI:

$ ftl
> call echo.echo
"Hello, Alice!"
> list
echo.echo
time.time
> profile list
local (local, default)
test (local)

It's not much work, basically just reusing the current CLI, including completion from #2707.

@stuartwdouglas had the additional idea that this could be automatically enabled for ftl dev, with a prompt always appearing at the bottom of the log output.