make echo const (#14997)

# Description

Make `echo` const.
- It's a very simple command, there is no reason for it to not be const.
- It's return type `any` is utilized in tests to type erase values, this
might be useful for testing const evaluation too.
- The upcoming custom command attribute feature can make use of it as a
stopgap replacement for `const def` commands.

# User-Facing Changes

`echo` can be used in const contexts.

# Tests + Formatting

# After Submitting
N/A

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

@@ -34,3 +34,9 @@ fn echo_range_handles_exclusive_down() {
 
     assert_eq!(actual.out, "[3,2]");
 }
+
+#[test]
+fn echo_is_const() {
+    let actual = nu!(r#"const val = echo 1..3; $val | to json --raw"#);
+    assert_eq!(actual.out, "[1,2,3]");
+}