Change the ignore command to use drain() instead of collecting a value (#12120)

# Description

Change the `ignore` command to use `drain()` instead of collecting a
value.

This saves memory usage when piping a lot of output to `ignore`. There's
no reason to keep the output in memory if it's going to be discarded
anyway.

# User-Facing Changes
Probably none

# Tests + Formatting
- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

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

@@ -0,0 +1,7 @@
+use nu_test_support::nu;
+
+#[test]
+fn ignore_still_causes_stream_to_be_consumed_fully() {
+    let result = nu!("[foo bar] | each { |val| print $val; $val } | ignore");
+    assert_eq!("foobar", result.out);
+}

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

@@ -45,6 +45,7 @@ mod hash_;
 mod headers;
 mod help;
 mod histogram;
+mod ignore;
 mod insert;
 mod inspect;
 mod interleave;