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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helps us keep track of breaking changes. -->

# Tests + Formatting
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> use toolkit.nu # or use an `env_change` hook to activate it
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src/main.rs

@@ -104,7 +104,9 @@ fn main() -> Result<()> {
                         default: nushell_config_path.display().to_string(),
                     },
                 );
-            } else if let Some(old_config) = nu_path::config_dir_old().map(|p| p.join("nushell")) {
+            } else if let Some(old_config) =
+                nu_path::get_canonicalized_path(dirs_next::config_dir()).map(|p| p.join("nushell"))
+            {
                 let xdg_config_empty = nushell_config_path
                     .read_dir()
                     .map_or(true, |mut dir| dir.next().is_none());
@@ -125,13 +127,22 @@ fn main() -> Result<()> {
         }
     }
 
+    let default_nushell_completions_path = if let Some(mut path) = nu_path::data_dir() {
+        path.push("nushell");
+        path.push("completions");
+        path
+    } else {
+        std::path::PathBuf::new()
+    };
+
     let mut default_nu_lib_dirs_path = nushell_config_path.clone();
     default_nu_lib_dirs_path.push("scripts");
     engine_state.add_env_var(
         "NU_LIB_DIRS".to_string(),
-        Value::test_list(vec![Value::test_string(
-            default_nu_lib_dirs_path.to_string_lossy(),
-        )]),
+        Value::test_list(vec![
+            Value::test_string(default_nu_lib_dirs_path.to_string_lossy()),
+            Value::test_string(default_nushell_completions_path.to_string_lossy()),
+        ]),
     );
 
     let mut default_nu_plugin_dirs_path = nushell_config_path;