Move most of the root package into a subcrate. (#1445)

This improves incremental build time when working on what was previously
the root package. For example, previously all plugins would be rebuilt
with a change to `src/commands/classified/external.rs`, but now only
`nu-cli` will have to be rebuilt (and anything that depends on it).

crates/nu-cli/tests/format_conversions/json.rs

@@ -0,0 +1,103 @@
+use nu_test_support::fs::Stub::FileWithContentToBeTrimmed;
+use nu_test_support::playground::Playground;
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn table_to_json_text_and_from_json_text_back_into_table() {
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            open sgml_description.json
+            | to-json
+            | from-json
+            | get glossary.GlossDiv.GlossList.GlossEntry.GlossSee
+            | echo $it
+        "#
+    ));
+
+    assert_eq!(actual, "markup");
+}
+
+#[test]
+fn from_json_text_to_table() {
+    Playground::setup("filter_from_json_test_1", |dirs, sandbox| {
+        sandbox.with_files(vec![FileWithContentToBeTrimmed(
+            "katz.txt",
+            r#"
+                {
+                    "katz": [
+                        {"name":   "Yehuda", "rusty_luck": 1},
+                        {"name": "Jonathan", "rusty_luck": 1},
+                        {"name":   "Andres", "rusty_luck": 1},
+                        {"name":"GorbyPuff", "rusty_luck": 1}
+                    ]
+                }
+            "#,
+        )]);
+
+        let actual = nu!(
+            cwd: dirs.test(),
+            "open katz.txt | from-json | get katz | get rusty_luck | count | echo $it"
+        );
+
+        assert_eq!(actual, "4");
+    })
+}
+
+#[test]
+fn from_json_text_recognizing_objects_independently_to_table() {
+    Playground::setup("filter_from_json_test_2", |dirs, sandbox| {
+        sandbox.with_files(vec![FileWithContentToBeTrimmed(
+            "katz.txt",
+            r#"
+                {"name":   "Yehuda", "rusty_luck": 1}
+                {"name": "Jonathan", "rusty_luck": 1}
+                {"name":   "Andres", "rusty_luck": 1}
+                {"name":"GorbyPuff", "rusty_luck": 3}
+            "#,
+        )]);
+
+        let actual = nu!(
+            cwd: dirs.test(), pipeline(
+            r#"
+                open katz.txt
+                | from-json -o
+                | where name == "GorbyPuff"
+                | get rusty_luck
+                | echo $it
+            "#
+        ));
+
+        assert_eq!(actual, "3");
+    })
+}
+
+#[test]
+fn table_to_json_text() {
+    Playground::setup("filter_to_json_test", |dirs, sandbox| {
+        sandbox.with_files(vec![FileWithContentToBeTrimmed(
+            "sample.txt",
+            r#"
+                JonAndrehudaTZ,3
+                GorbyPuff,100
+            "#,
+        )]);
+
+        let actual = nu!(
+            cwd: dirs.test(), pipeline(
+            r#"
+                open sample.txt
+                | lines
+                | split-column "," name luck
+                | pick name
+                | to-json
+                | from-json
+                | nth 0
+                | get name
+                | echo $it
+            "#
+        ));
+
+        assert_eq!(actual, "JonAndrehudaTZ");
+    })
+}