Pipeline operators: && and || (#7448)

# Description

We got some feedback from folks used to other shells that `try/catch`
isn't quite as convenient as things like `||`. This PR adds `&&` as a
synonym for `;` and `||` as equivalent to what `try/catch` would do.

# User-Facing Changes

Adds `&&` and `||` pipeline operators.

# 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 -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# 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-protocol/src/engine/stack.rs

@@ -113,6 +113,13 @@ impl Stack {
             .ok_or_else(|| ShellError::NushellFailed("No active overlay".into()))
     }
 
+    pub fn last_exit_code(&self, engine_state: &EngineState) -> Option<i64> {
+        match self.get_env_var(engine_state, "LAST_EXIT_CODE") {
+            Some(Value::Int { val, .. }) => Some(val),
+            _ => None,
+        }
+    }
+
     pub fn captures_to_stack(&self, captures: &HashMap<VarId, Value>) -> Stack {
         // FIXME: this is probably slow
         let mut env_vars = self.env_vars.clone();