to json -r not removing whitespaces fix (#11948)

fixes #11900  

# Description
Use `serde_json` instead.

# User-Facing Changes
The problem described in the issue now no longer persists.

No whitespace in the output of `to json --raw`
Output of unicode escape changed to consistent `\uffff`

# Tests + Formatting
I corrected all Tests that were affected by this change.

crates/nu-command/tests/commands/each.rs

@@ -32,7 +32,7 @@ fn each_window_stride() {
 fn each_no_args_in_block() {
     let actual = nu!("echo [[foo bar]; [a b] [c d] [e f]] | each {|i| $i | to json -r } | get 1");
 
-    assert_eq!(actual.out, r#"{"foo": "c","bar": "d"}"#);
+    assert_eq!(actual.out, r#"{"foo":"c","bar":"d"}"#);
 }
 
 #[test]