Uniq: --count flag to count occurences (#2017)

* uniq: Add counting option (WIP!)

Usage:

fetch https://raw.githubusercontent.com/timbray/topfew/master/test/data/access-1k | lines | wrap item | uniq | sort-by count | last 10

* uniq: Add first test

* uniq: Re-enable the non-counting variant.

* uniq: Also handle primitive lines.

* uniq: Update documentation

* uniq: Final comment about error handling. Let's get some feedback

* uniq: Address review comments.

Not happy with the way I create a TypeError. There must be a cleaner
way. Anyway, good for shipping.

* uniq: Use Labeled_error as suggested by jturner in chat.

* uniq: Return error directly.

Co-authored-by: Christoph Siedentop <christoph@siedentop.name>

crates/nu-cli/tests/commands/uniq.rs

@@ -140,3 +140,26 @@ fn uniq_when_keys_out_of_order() {
 
     assert_eq!(actual.out, "1");
 }
+
+#[test]
+fn uniq_counting() {
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            echo '["A", "B", "A"]'
+            | from json
+            | wrap item
+            | uniq --count
+        "#
+    ));
+    let expected = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+        echo '[{"item": "A", "count": 2}, {"item": "B", "count": 1}]'
+        | from json
+        "#
+    ));
+    print!("{}", actual.out);
+    print!("{}", expected.out);
+    assert_eq!(actual.out, expected.out);
+}