Allow dropping columns. (#3107)

`drop` is used for removing the last row. Passing a number allows dropping N rows.
Here we introduce the same logic for dropping columns instead.

You can certainly remove columns by using `reject`, however, there could be cases
where we are interested in removing columns from tables that contain, say, a big
number of columns. Using `reject` becomes impractical, especially when you don't
care about the column names that could either be known or not known when exploring
tables.

```
> echo [[lib, extension]; [nu-core, rs] [rake, rb]]
─────────┬───────────
   lib   │ extension
─────────┼───────────
 nu-core │ rs
 rake    │ rb
─────────┴───────────
```

```
> echo [[lib, extension]; [nu-core, rs] [rake, rb]] | drop column
─────────
   lib
─────────
 nu-core
 rake
─────────
```

crates/nu-command/tests/commands/drop.rs

@@ -1,23 +1,68 @@
 use nu_test_support::{nu, pipeline};
 
 #[test]
-fn drop_rows() {
+fn columns() {
     let actual = nu!(
-        cwd: "tests/fixtures/formats",
-        r#"echo '[{"foo": 3}, {"foo": 8}, {"foo": 4}]' | from json | drop 2 | get foo | math sum "#
+        cwd: ".", pipeline(r#"
+            echo [
+              [arepas, color];
+
+              [3,  white]
+              [8, yellow]
+              [4,  white]
+            ]
+            | drop column
+            | get
+            | count
+        "#)
+    );
+
+    assert_eq!(actual.out, "1");
+}
+
+#[test]
+fn more_columns_than_table_has() {
+    let actual = nu!(
+        cwd: ".", pipeline(r#"
+            echo [
+              [arepas, color];
+
+              [3,  white]
+              [8, yellow]
+              [4,  white]
+            ]
+            | drop column 3
+            | get
+            | empty?
+        "#)
+    );
+
+    assert_eq!(actual.out, "true");
+}
+
+#[test]
+fn rows() {
+    let actual = nu!(
+        cwd: ".", pipeline(r#"
+            echo [
+              [arepas];
+
+              [3]
+              [8]
+              [4]
+            ]
+            | drop 2
+            | get arepas
+            | math sum
+        "#)
     );
 
     assert_eq!(actual.out, "3");
 }
 
 #[test]
-fn drop_more_rows_than_table_has() {
-    let actual = nu!(
-        cwd: ".", pipeline(
-        r#"
-        date | drop 50 | count
-        "#
-    ));
+fn more_rows_than_table_has() {
+    let actual = nu!(cwd: ".", "date | drop 50 | count");
 
     assert_eq!(actual.out, "0");
 }