Canonicalize config dir

[ysthakur]

Description

It turns out that my previous PR, #11999, didn't properly canonicalize $nu.default-config-dir in a scenario where XDG_CONFIG_HOME (or the equivalent on each platform) was a symlink. To remedy that, this PR makes nu_path::config_dir() return a canonicalized path. This probably shouldn't break anything (except maybe tests relying on the old behavior), since the canonical path will be equivalent to non-canonical paths.

User-Facing Changes

A user may get a path with symlinks resolved and ..s replaced where they previously didn't. I'm not sure where this would happen, though, and anyway, the canonical path is probably the "correct" thing to present to the user. We're using omnipath to make the path presentable to the user on Windows, so there's no danger of someone getting an path with \\? there.

Tests + Formatting

The tests for config files have been updated to run the binary using the Director so that it has access to the XDG_CONFIG_HOME/HOME environment variables to be able to change the config directory.

After Submitting

[sholderbach]

Thanks! This looks good to me.

Ah the sweet difference between nu! and run... (Correction your run is the secret third thing 😄)