Use XDG_CONFIG_HOME before default config directory (#12118)

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Closes #12103

# Description
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As described in #12103, this PR makes Nushell use `XDG_CONFIG_HOME` as
the config directory if it exists. Otherwise, it uses the old behavior,
which was to use `dirs_next::config_dir()`.

Edit: We discussed choosing between `XDG_CONFIG_HOME` and the default
config directory in Discord and decided against it, at least for now.

<s>@kubouch also suggested letting users choose between
`XDG_CONFIG_HOME` and the default config directory if config files
aren't found on startup and `XDG_CONFIG_HOME` is set to a value
different from the default config directory</s>

On Windows and MacOS, if the `XDG_CONFIG_HOME` variable is set but
`XDG_CONFIG_HOME` is either empty or doesn't exist *and* the old config
directory is non-empty, Nushell will issue a warning on startup saying
that it won't move files from the old config directory to the new one.
To do this, I had to add a `nu_path::config_dir_old()` function. I
assume that at some point, we will remove the warning message and the
function can be removed too. Alternatively, instead of having that
function there, `main.rs` could directly call `dirs_next::config_dir()`.

# User-Facing Changes
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When `$env.XDG_CONFIG_HOME` is set to an absolute path, Nushell will use
`$"($env.XDG_CONFIG_HOME)/nushell"` as its config directory (previously,
this only worked on Linux).

To use `App Data\Roaming` (Windows) or `Library/Application Support`
(MacOS) instead (the old behavior), one can either leave
`XDG_CONFIG_HOME` unset or set it to an empty string.

If `XDG_CONFIG_HOME` is set, but to a non-absolute/invalid path, Nushell
will report an error on startup and use the default config directory
instead:


![image](https://github.com/nushell/nushell/assets/45539777/a434fe04-b7c8-4e95-b50c-80628008ad08)

On Windows and MacOS, if the `XDG_CONFIG_HOME` variable is set but
`XDG_CONFIG_HOME` is either empty or doesn't exist *and* the old config
directory is non-empty, Nushell will issue a warning on startup saying
that it won't move files from the old config directory to the new one.


![image](https://github.com/nushell/nushell/assets/45539777/1686cc17-4083-4c12-aecf-1d832460ca57)


# Tests + Formatting
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- `cargo run -- -c "use std testing; testing run-tests --path
crates/nu-std"` to run the tests for the standard library

> **Note**
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The existing config path tests have been modified to use
`XDG_CONFIG_HOME` to change the config directory on all OSes, not just
Linux.

# After Submitting
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The documentation will have to be updated to note that Nushell uses
`XDG_CONFIG_HOME` now. As @fdncred pointed out, it's possible for people
to set `XDG_CONFIG_HOME` to, say, `~/.config/nushell` rather than
`~/.config`, so the documentation could warn about that mistake.

crates/nu-path/src/helpers.rs

@@ -7,7 +7,18 @@ pub fn home_dir() -> Option<PathBuf> {
 }
 
 pub fn config_dir() -> Option<PathBuf> {
-    dirs_next::config_dir().map(|path| canonicalize(&path).unwrap_or(path))
+    match std::env::var("XDG_CONFIG_HOME").map(PathBuf::from) {
+        Ok(xdg_config) if xdg_config.is_absolute() => {
+            Some(canonicalize(&xdg_config).unwrap_or(xdg_config))
+        }
+        _ => config_dir_old(),
+    }
+}
+
+/// Get the old default config directory. Outside of Linux, this will ignore `XDG_CONFIG_HOME`
+pub fn config_dir_old() -> Option<PathBuf> {
+    let path = dirs_next::config_dir()?;
+    Some(canonicalize(&path).unwrap_or(path))
 }
 
 #[cfg(windows)]

crates/nu-path/src/lib.rs

@@ -5,6 +5,6 @@ mod tilde;
 mod util;
 
 pub use expansions::{canonicalize_with, expand_path_with, expand_to_real_path};
-pub use helpers::{config_dir, home_dir};
+pub use helpers::{config_dir, config_dir_old, home_dir};
 pub use tilde::expand_tilde;
 pub use util::trim_trailing_slash;

crates/nu-protocol/src/errors/shell_error.rs

@@ -1341,6 +1341,14 @@ On Windows, this would be %USERPROFILE%\AppData\Roaming"#
         #[label = "Could not find config directory"]
         span: Option<Span>,
     },
+
+    /// XDG_CONFIG_HOME was set to an invalid path
+    #[error("$env.XDG_CONFIG_HOME ({xdg}) is invalid, using default config directory instead: {default}")]
+    #[diagnostic(
+        code(nu::shell::xdg_config_home_invalid),
+        help("Set XDG_CONFIG_HOME to an absolute path, or set it to an empty string to ignore it")
+    )]
+    InvalidXdgConfig { xdg: String, default: String },
 }
 
 // TODO: Implement as From trait