nushell/post.org

# WIP: Per directory env-variables
For #86

Environment variables are added if you have created a file called .nu inside a whitelisted directory, formatted as shown below. (I am, of course, open to change everything about this)
```
[env]
var = "value"
anothervar = "anothervalue"
```

In order for a .nu-file to be read, the directory it is in must be listed in the `nu_env_dirs` variable in nushell's `config.toml`.
```
nu_env_dirs = ["/home/sam", "/home/sam/github", "/home/sam/github/test"]
```
This was implemented for the sake of security. I do not believe that it is appropiate for nushell to pick up random .nu-files unless
the user has explicitly said that it's alright. An adversary could hide a .nu-file in an otherwise unsuspicous folder that you download,
and now you suddenly have nushell picking up whatever environment variables they set.
This meant that the code necessarily become more involved than just looking for a .nu-file in the current directory and applying its variables,
but this extra code also allowed for some other nice features and behavior which is listed below.

Behavior:
- If you are in a subdirectory to a directory with a .nu-file, the vars in that .nu-file are applied.
- If you leave a directory which set some variables, the variables are unset.
- If a directory contains a .nu with an environment variable already set, the old value will be overwritten with the value from the .nu. This holds even if the old value was set by a .nu in a parent directory.
  - The overwritten value is restored when you leave the directory.
  - TODO: What happens if you overwrite twice?

Questions:
`remove_env()` accesses the environment directly to remove variables.
Just using what I think was expected, envs.entries.remove(key), does not work.
If this is a problem, how can I make it work better with existing code?

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