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-protocol/src/lib.rs

@@ -25,6 +25,7 @@ use nu_protocol::{
     ast::Operator, engine::Closure, ByteStreamType, Config, LabeledError, PipelineData,
     PluginMetadata, PluginSignature, ShellError, Span, Spanned, Value,
 };
+use nu_utils::SharedCow;
 use serde::{Deserialize, Serialize};
 use std::collections::HashMap;
 
@@ -553,7 +554,7 @@ impl<D> EngineCall<D> {
 pub enum EngineCallResponse<D> {
     Error(ShellError),
     PipelineData(D),
-    Config(Box<Config>),
+    Config(SharedCow<Config>),
     ValueMap(HashMap<String, Value>),
 }