add $nu.data-dir for completions and $nu.cache-dir for other uses (#13122)

# Description

This PR is an attempt to add a standard location for people to put
completions in. I saw this topic come up again recently and IIRC we
decided to create a standard location. I used the dirs-next crate to
dictate where these locations are. I know some people won't like that
but at least this gets the ball rolling in a direction that has a
standard directory.

This is what the default NU_LIB_DIRS looks like now in the
default_env.nu. It should also be like this when starting nushell with
`nu -n`
```nushell
$env.NU_LIB_DIRS = [
    ($nu.default-config-dir | path join 'scripts') # add <nushell-config-dir>/scripts
    ($nu.data-dir | path join 'completions') # default home for nushell completions
]
```

I also added these default folders to the `$nu` variable so now there is
`$nu.data-path` and `$nu.cache-path`.

## Data Dir Default

![image](https://github.com/nushell/nushell/assets/343840/aeeb7cd6-17b4-43e8-bb6f-986a0c7fce23)

While I was in there, I also decided to add a cache dir

## Cache Dir Default

![image](https://github.com/nushell/nushell/assets/343840/87dead66-4911-4f67-bfb2-acb16f386674)

### This is what the default looks like in Ubuntu.

![image](https://github.com/nushell/nushell/assets/343840/bca8eae8-8c18-47e8-b64f-3efe34f0004f)

### This is what it looks like with XDG_CACHE_HOME and XDG_DATA_HOME
overridden
```nushell
XDG_DATA_HOME=/tmp/data_home XDG_CACHE_HOME=/tmp/cache_home cargo r
```

![image](https://github.com/nushell/nushell/assets/343840/fae86d50-9821-41f1-868e-3814eca3730b)

### This is what the defaults look like in Windows (username scrubbed to
protect the innocent)

![image](https://github.com/nushell/nushell/assets/343840/3ebdb5cd-0150-448c-aff5-c57053e4788a)

How my NU_LIB_DIRS is set in the images above
```nushell
$env.NU_LIB_DIRS = [
    ($nu.default-config-dir | path join 'scripts') # add <nushell-config-dir>/scripts
    '/Users/fdncred/src/nu_scripts'
    ($nu.config-path | path dirname)
    ($nu.data-dir | path join 'completions') # default home for nushell completions
]
```

Let the debate begin.

# User-Facing Changes
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crates/nu-path/src/helpers.rs

@@ -6,18 +6,36 @@ pub fn home_dir() -> Option<PathBuf> {
     dirs_next::home_dir()
 }
 
+/// Return the data directory for the current platform or XDG_DATA_HOME if specified.
+pub fn data_dir() -> Option<PathBuf> {
+    match std::env::var("XDG_DATA_HOME").map(PathBuf::from) {
+        Ok(xdg_data) if xdg_data.is_absolute() => Some(canonicalize(&xdg_data).unwrap_or(xdg_data)),
+        _ => get_canonicalized_path(dirs_next::data_dir()),
+    }
+}
+
+/// Return the cache directory for the current platform or XDG_CACHE_HOME if specified.
+pub fn cache_dir() -> Option<PathBuf> {
+    match std::env::var("XDG_CACHE_HOME").map(PathBuf::from) {
+        Ok(xdg_cache) if xdg_cache.is_absolute() => {
+            Some(canonicalize(&xdg_cache).unwrap_or(xdg_cache))
+        }
+        _ => get_canonicalized_path(dirs_next::cache_dir()),
+    }
+}
+
+/// Return the config directory for the current platform or XDG_CONFIG_HOME if specified.
 pub fn config_dir() -> Option<PathBuf> {
     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_canonicalized_path(dirs_next::config_dir()),
     }
 }
 
-/// 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()?;
+pub fn get_canonicalized_path(path: Option<PathBuf>) -> Option<PathBuf> {
+    let path = path?;
     Some(canonicalize(&path).unwrap_or(path))
 }
 

crates/nu-path/src/lib.rs

@@ -8,6 +8,6 @@ mod trailing_slash;
 
 pub use components::components;
 pub use expansions::{canonicalize_with, expand_path_with, expand_to_real_path, locate_in_dirs};
-pub use helpers::{config_dir, config_dir_old, home_dir};
+pub use helpers::{cache_dir, config_dir, data_dir, get_canonicalized_path, home_dir};
 pub use tilde::expand_tilde;
 pub use trailing_slash::{has_trailing_slash, strip_trailing_slash};