Update config directly at assignment (#13332)

# Description

Allows `Stack` to have a modified local `Config`, which is updated
immediately when `$env.config` is assigned to. This means that even
within a script, commands that come after `$env.config` changes will
always see those changes in `Stack::get_config()`.

Also fixed a lot of cases where `engine_state.get_config()` was used
even when `Stack` was available.

Closes #13324.

# User-Facing Changes
- Config changes apply immediately after the assignment is executed,
rather than whenever config is read by a command that needs it.
- Potentially slower performance when executing a lot of lines that
change `$env.config` one after another. Recommended to get `$env.config`
into a `mut` variable first and do modifications, then assign it back.
- Much faster performance when executing a script that made
modifications to `$env.config`, as the changes are only parsed once.

# Tests + Formatting
All passing.

# After Submitting
- [ ] release notes

crates/nu-cli/src/repl.rs

@@ -297,7 +297,7 @@ fn loop_iteration(ctx: LoopContext) -> (bool, Stack, Reedline) {
     perf!("env-change hook", start_time, use_color);
 
     let engine_reference = Arc::new(engine_state.clone());
-    let config = engine_state.get_config();
+    let config = stack.get_config(engine_state);
 
     start_time = std::time::Instant::now();
     // Find the configured cursor shapes for each mode
@@ -323,7 +323,6 @@ fn loop_iteration(ctx: LoopContext) -> (bool, Stack, Reedline) {
             engine_state: engine_reference.clone(),
             // STACK-REFERENCE 1
             stack: stack_arc.clone(),
-            config: config.clone(),
         }))
         .with_validator(Box::new(NuValidator {
             engine_state: engine_reference.clone(),