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/Cargo.toml

@@ -15,6 +15,7 @@ nu-engine = { path = "../nu-engine", version = "0.95.1" }
 nu-protocol = { path = "../nu-protocol", version = "0.95.1" }
 nu-plugin-protocol = { path = "../nu-plugin-protocol", version = "0.95.1" }
 nu-plugin-core = { path = "../nu-plugin-core", version = "0.95.1", default-features = false }
+nu-utils = { path = "../nu-utils", version = "0.95.1" }
 
 log = { workspace = true }
 thiserror = "1.0"
@@ -29,4 +30,4 @@ local-socket = ["nu-plugin-core/local-socket"]
 
 [target.'cfg(target_family = "unix")'.dependencies]
 # For setting the process group ID (EnterForeground / LeaveForeground)
-nix = { workspace = true, default-features = false, features = ["process"] }
+nix = { workspace = true, default-features = false, features = ["process"] }

crates/nu-plugin/src/plugin/interface/mod.rs

@@ -14,6 +14,7 @@ use nu_protocol::{
     engine::Closure, Config, LabeledError, PipelineData, PluginMetadata, PluginSignature,
     ShellError, Signals, Span, Spanned, Value,
 };
+use nu_utils::SharedCow;
 use std::{
     collections::{btree_map, BTreeMap, HashMap},
     sync::{mpsc, Arc},
@@ -525,9 +526,9 @@ impl EngineInterface {
     /// # Ok(())
     /// # }
     /// ```
-    pub fn get_config(&self) -> Result<Box<Config>, ShellError> {
+    pub fn get_config(&self) -> Result<Arc<Config>, ShellError> {
         match self.engine_call(EngineCall::GetConfig)? {
-            EngineCallResponse::Config(config) => Ok(config),
+            EngineCallResponse::Config(config) => Ok(SharedCow::into_arc(config)),
             EngineCallResponse::Error(err) => Err(err),
             _ => Err(ShellError::PluginFailedToDecode {
                 msg: "Received unexpected response for EngineCall::GetConfig".into(),

crates/nu-plugin/src/plugin/mod.rs

@@ -670,7 +670,7 @@ fn print_help(plugin: &impl Plugin, encoder: impl PluginEncoder) {
                 }
             })
             .and_then(|_| {
-                let flags = get_flags_section(None, &signature, |v| format!("{:#?}", v));
+                let flags = get_flags_section(None, None, &signature, |v| format!("{:#?}", v));
                 write!(help, "{flags}")
             })
             .and_then(|_| writeln!(help, "\nParameters:"))