add to ndjson and to jsonl to the standard library (#10519)

follow up to
- #10283

# Description
even though it appears defining `to foo` does not allow to do `save
x.foo` for free (see https://github.com/nushell/nushell/issues/10429),
because #10283 did add `from ndjson` and `from jsonl` to the standard
library, i thought adding their `to ...` counterpart would make sense
😋

# User-Facing Changes
users can now convert structured data back to NDJSON and JSONL 👌

# Tests + Formatting
this PR adds the exact same tests as for the `from ...` commands
- structured data is in `result` and the string is now the expected
- the two invalid `from ...` tests cannot be reproduced for `to ...`
afaik

# After Submitting

crates/nu-std/std/formats.nu

@@ -9,12 +9,22 @@
 # These functions help `open` the files with unsupported extensions such as ndjson.
 #
 
-# Convert from ndjson to structured data.
+# Convert from [NDJSON](http://ndjson.org/) to structured data.
 export def "from ndjson" []: string -> any {
     from json --objects
 }
 
-# Convert from jsonl to structured data.
+# Convert from [JSONL](https://jsonlines.org/) to structured data.
 export def "from jsonl" []: string -> any {
     from json --objects
 }
+
+# Convert structured data to [NDJSON](http://ndjson.org/).
+export def "to ndjson" []: any -> string {
+    each { to json --raw } | to text
+}
+
+# Convert structured data to [JSONL](https://jsonlines.org/).
+export def "to jsonl" []: any -> string {
+    each { to json --raw } | to text
+}

crates/nu-std/tests/test_formats.nu

@@ -1,18 +1,26 @@
 use std assert
 
-def ndjson_test_data1 [] {
-  '{"a":1}
-{"a":2}
-{"a":3}
-{"a":4}
-{"a":5}
-{"a":6}'
+def test_data_multiline [] {
+  let lines = [
+    "{\"a\": 1}",
+    "{\"a\": 2}",
+    "{\"a\": 3}",
+    "{\"a\": 4}",
+    "{\"a\": 5}",
+    "{\"a\": 6}",
+  ]
+
+  if $nu.os-info.name == "windows" {
+    $lines | str join "\r\n"
+  } else {
+    $lines | str join "\n"
+  }
 }
 
 #[test]
 def from_ndjson_multiple_objects [] {
   use std formats *
-  let result = ndjson_test_data1 | from ndjson
+  let result = test_data_multiline | from ndjson
   let expect = [{a:1},{a:2},{a:3},{a:4},{a:5},{a:6}]
   assert equal $result $expect "could not convert from NDJSON"
 }
@@ -34,7 +42,7 @@ def from_ndjson_invalid_object [] {
 #[test]
 def from_jsonl_multiple_objects [] {
   use std formats *
-  let result = ndjson_test_data1 | from jsonl
+  let result = test_data_multiline | from jsonl
   let expect = [{a:1},{a:2},{a:3},{a:4},{a:5},{a:6}]
   assert equal $result $expect "could not convert from JSONL"
 }
@@ -52,3 +60,35 @@ def from_jsonl_invalid_object [] {
   use std formats *
   assert error { '{"a":1' | from jsonl }
 }
+
+#[test]
+def to_ndjson_multiple_objects [] {
+  use std formats *
+  let result = [{a:1},{a:2},{a:3},{a:4},{a:5},{a:6}] | to ndjson | str trim
+  let expect = test_data_multiline
+  assert equal $result $expect "could not convert to NDJSON"
+}
+
+#[test]
+def to_ndjson_single_object [] {
+  use std formats *
+  let result = [{a:1}] | to ndjson | str trim
+  let expect = "{\"a\": 1}"
+  assert equal $result $expect "could not convert to NDJSON"
+}
+
+#[test]
+def to_jsonl_multiple_objects [] {
+  use std formats *
+  let result = [{a:1},{a:2},{a:3},{a:4},{a:5},{a:6}] | to jsonl | str trim
+  let expect = test_data_multiline
+  assert equal $result $expect "could not convert to JSONL"
+}
+
+#[test]
+def to_jsonl_single_object [] {
+  use std formats *
+  let result = [{a:1}] | to jsonl | str trim
+  let expect = "{\"a\": 1}"
+  assert equal $result $expect "could not convert to JSONL"
+}