option to debug variables, includes, env files.

`just` becames my main task manager for local (and CI) development from more than 1 year, and I often have the some profound questions around development lifecycle, and more precisely in this moment I think about these ones:
- which is the .env used file that breaks my previously perfectly working project
- why `this_variable` is not set like I expect, some include might rework it and i do not know it
- in which include or module is configured the target `this_great_target`.

`just --verbose` display me the target name it calls, but i would like to have a bit more than that ! In `-v` parameter, or with `--debug`:

- print the dependency tree (like [mise tasks deps](https://mise.jdx.dev/cli/tasks/deps.html]))
- which .env file has been loaded (with full path)
- for each called target, in which file + line it has been defined
- that would be great to have the same for variables, but a dump of all defined variables (+ which files they has been set) would be very great !

Thanks a lot