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/path/exists.rs

@@ -57,3 +57,9 @@ fn checks_tilde_relative_path_exists() {
     let actual = nu!("'~' | path exists");
     assert_eq!(actual.out, "true");
 }
+
+#[test]
+fn const_path_exists() {
+    let actual = nu!("const exists = ('~' | path exists); $exists");
+    assert_eq!(actual.out, "true");
+}