ansi name for clear-scrollback code (#14184)

Related to #14181

# Description

Our understanding of `ESC[3J` has apparently been wrong. And I say "our"
because I posted a [Super User
answer](https://superuser.com/a/1738611/1210833) a couple of years ago
with the same misconception (now fixed). In addition, the [crossterm
crate
doc](https://docs.rs/crossterm/latest/crossterm/terminal/enum.ClearType.html)
is wrong on the topic.

`ESC[3J` doesn't clear the screen plus the scrollback; it *only* clears
the scrollback. Reference the official [Xterm Control Sequences
doc](https://www.xfree86.org/4.8.0/ctlseqs.html).

> CSI P s J
> 
> Erase in Display (ED)
> 
> P s = 0 → Erase Below (default)
> P s = 1 → Erase Above
> P s = 2 → Erase All
> P s = 3 → Erase Saved Lines (xterm)

This also means that:

```nu
$"(ansi clear_entire_screen_plus_buffer)"
```

... doesn't.

This PR updates it to `ansi clear_scrollback_buffer` (short-code remains
the same).

# User-Facing Changes

Breaking-change: `ansi clear_entire_screen_plus_buffer` is renamed `ansi
clear_scrollback_buffer`

# Tests + Formatting

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting

Self-documenting command via `ansi -l`

crates/nu-command/src/platform/ansi/ansi_.rs

@@ -448,7 +448,8 @@ static CODE_LIST: Lazy<Vec<AnsiCode>> = Lazy::new(|| { vec![
     AnsiCode{ short_name: None, long_name:"clear_screen_from_cursor_to_end", code: "\x1b[0J".to_string()}, // clears from cursor until end of screen
     AnsiCode{ short_name: None, long_name:"clear_screen_from_cursor_to_beginning", code: "\x1b[1J".to_string()}, // clears from cursor to beginning of screen
     AnsiCode{ short_name: Some("cls"), long_name:"clear_entire_screen", code: "\x1b[2J".to_string()}, // clears the entire screen
-    AnsiCode{ short_name: Some("clsb"), long_name:"clear_entire_screen_plus_buffer", code: "\x1b[3J".to_string()}, // clear entire screen and delete all lines saved in the scrollback buffer
+    AnsiCode{ short_name: Some("clsb"), long_name:"clear_entire_screen_plus_buffer", code: "\x1b[2J\x1b[3J".to_string()}, // clear entire screen and delete all lines saved in the scrollback buffer
+    AnsiCode{ short_name: Some("clb"), long_name:"clear_scrollback_buffer", code: "\x1b[3J".to_string()}, // clear entire screen and delete all lines saved in the scrollback buffer
     AnsiCode{ short_name: None, long_name:"erase_line", code: "\x1b[K".to_string()},                   // clears the current line
     AnsiCode{ short_name: None, long_name:"erase_line_from_cursor_to_end", code: "\x1b[0K".to_string()}, // clears from cursor to end of line
     AnsiCode{ short_name: None, long_name:"erase_line_from_cursor_to_beginning", code: "\x1b[1K".to_string()}, // clears from cursor to start of line

crates/nu-command/tests/commands/platform/ansi_.rs

@@ -11,5 +11,19 @@ fn test_ansi_shows_error_on_escape() {
 fn test_ansi_list_outputs_table() {
     let actual = nu!("ansi --list | length");
 
-    assert_eq!(actual.out, "424");
+    assert_eq!(actual.out, "425");
+}
+
+#[test]
+fn test_ansi_codes() {
+    let actual = nu!("$'(ansi clear_scrollback_buffer)'");
+    assert_eq!(actual.out, "\x1b[3J");
+
+    // Currently, bg is placed before fg in the results
+    // It's okay if something internally changes this, but
+    // if so, the test case will need to be updated to:
+    // assert_eq!(actual.out, "\x1b[31;48;2;0;255;0mHello\x1b[0m");
+
+    let actual = nu!("$'(ansi { fg: red, bg: \"#00ff00\" })Hello(ansi reset)'");
+    assert_eq!(actual.out, "\x1b[48;2;0;255;0;31mHello\x1b[0m");
 }