Allow parse-time evaluation of calls, pipelines and subexpressions (#9499)

Co-authored-by: Antoine Stevan <44101798+amtoine@users.noreply.github.com>

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_const() {
+    let actual = nu!("const x = (echo spam); $x");
+    assert_eq!(actual.out, "spam");
+}