Simplify expressions (#3389)

* WIP: experiment with simpler expressions

* fix simple invoke

* update tests

* fix a few tests

* Make paren parsing more robust

* fix external args

* Remove old invocation

* Update tests

* Update tests

crates/nu-cli/src/completion/engine.rs

@@ -384,7 +384,7 @@ mod tests {
         #[test]
         fn completes_incomplete_nested_structure() {
             let registry: VecRegistry = vec![Signature::build("sys")].into();
-            let line = "echo $(sy";
+            let line = "echo (sy";
 
             assert_eq!(
                 completion_location(line, &registry, 8),

crates/nu-cli/src/shell/completer.rs

@@ -145,7 +145,7 @@ fn requote(orig_value: String) -> String {
     if should_quote {
         if quotes.is_empty() {
             // TODO we don't really have an escape character, so there isn't a great option right
-            //      now. One possibility is `{{$(char backtick)}}`
+            //      now. One possibility is `{{(char backtick)}}`
             value.to_string()
         } else {
             let quote = quotes[0];