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-plugin-engine/src/interface/mod.rs

@@ -14,6 +14,7 @@ use nu_protocol::{
     ast::Operator, CustomValue, IntoSpanned, PipelineData, PluginMetadata, PluginSignature,
     ShellError, Signals, Span, Spanned, Value,
 };
+use nu_utils::SharedCow;
 use std::{
     collections::{btree_map, BTreeMap},
     sync::{mpsc, Arc, OnceLock},
@@ -1260,7 +1261,7 @@ pub(crate) fn handle_engine_call(
 
     match call {
         EngineCall::GetConfig => {
-            let config = Box::new(context.get_config()?);
+            let config = SharedCow::from(context.get_config()?);
             Ok(EngineCallResponse::Config(config))
         }
         EngineCall::GetPluginConfig => {