feat: ✨ (stdlib) add os record support to path add (#9238)

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Add "os record" support.


# User-Facing Changes
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This don't change how path add works but just adds support for "os
records" aka records whose key are at least one of:
- linux
- macos
- windows

Check the
[test](a917f1a924/crates/nu-std/tests/test_std.nu (L31-L32))
if that's not clear enough

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---------

Co-authored-by: amtoine <stevan.antoine@gmail.com>

crates/nu-std/tests/test_std.nu

@@ -24,6 +24,14 @@ export def test_path_add [] {
 
         assert equal (std path add "fooooo" --ret) ["fooooo", "foo", "bar", "baz"]
         assert equal (get_path) ["fooooo", "foo", "bar", "baz"]
+
+        let-env $path_name = []
+        let target_paths = {linux: "foo", windows: "bar", macos: "baz"}
+        
+        std path add $target_paths
+        assert equal (get_path) [($target_paths | get $nu.os-info.name)]
+
+        
     }
 }