Change echo to print when not redirected (#10338)

# Description

This changes `echo` to work more closely to what users of other shells
would expect:

* when redirected, `echo` works as before and sends values through the
pipeline
* when not redirected, `echo` will print values to the screen/terminal

# User-Facing Changes

A standalone `echo` now will print to the terminal, if not redirected.

The `echo` command is no longer const eval-able, as it will now print to
the terminal in some cases.

# Tests + Formatting
<!--
Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used` to
check that you're using the standard code style
- `cargo test --workspace` to check that all tests pass (on Windows make
sure to [enable developer
mode](https://learn.microsoft.com/en-us/windows/apps/get-started/developer-mode-features-and-debugging))
- `cargo run -- -c "use std testing; testing run-tests --path
crates/nu-std"` to run the tests for the standard library

> **Note**
> from `nushell` you can also use the `toolkit` as follows
> ```bash
> use toolkit.nu # or use an `env_change` hook to activate it
automatically
> toolkit check pr
> ```
-->

# After Submitting
<!-- If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.
-->

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

@@ -34,9 +34,3 @@ fn echo_range_handles_exclusive_down() {
 
     assert_eq!(actual.out, "[3,2]");
 }
-
-#[test]
-fn echo_const() {
-    let actual = nu!("const x = (echo spam); $x");
-    assert_eq!(actual.out, "spam");
-}

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

@@ -4,7 +4,7 @@ use nu_test_support::nu;
 fn for_doesnt_auto_print_in_each_iteration() {
     let actual = nu!("
         for i in 1..2 {
-            echo 1
+            $i
         }");
     // Make sure we don't see any of these values in the output
     // As we do not auto-print loops anymore

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

@@ -10,7 +10,7 @@ fn loop_doesnt_auto_print_in_each_iteration() {
             } else {
                 $total += 1;
             }
-            echo 1
+            1
         }");
     // Make sure we don't see any of these values in the output
     // As we do not auto-print loops anymore

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

@@ -11,7 +11,7 @@ fn while_sum() {
 
 #[test]
 fn while_doesnt_auto_print_in_each_iteration() {
-    let actual = nu!("mut total = 0; while $total < 2 { $total = $total + 1; echo 1 }");
+    let actual = nu!("mut total = 0; while $total < 2 { $total = $total + 1; 1 }");
     // Make sure we don't see any of these values in the output
     // As we do not auto-print loops anymore
     assert!(!actual.out.contains('1'));