Add serialization for JSON and form bodies in post (#4764)

* Add serialization for JSON and form bodies in `post`

* Reuse code from `to json` instead of duplicating

* Fix formatting. Oops

crates/nu-command/src/formats/to/mod.rs

@@ -22,3 +22,5 @@ pub use nuon::ToNuon;
 pub use tsv::ToTsv;
 pub use xml::ToXml;
 pub use yaml::ToYaml;
+
+pub(crate) use json::value_to_json_value;