fix multiline strings in NDNUON (#14519)

- should close https://github.com/nushell/nushell/issues/14517

# Description
this will change `to ndnuon` so that newlines are encoded as a literal
`\n` which `from ndnuon` is already able to handle

# User-Facing Changes
users should be able to encode multiline strings in NDNUON

# Tests + Formatting
new tests have been added:
- they don't pass on the first commit
- they do pass with the fix

# After Submitting

crates/nu-std/std/formats/mod.nu

@@ -36,5 +36,5 @@ export def "from ndnuon" []: [string -> any] {
 
 # Convert structured data to NDNUON, i.e. newline-delimited NUON
 export def "to ndnuon" []: [any -> string] {
-    each { to nuon --raw } | to text
+    each { to nuon --raw | str replace --all "\n" '\n' } | to text
 }