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-command/src/conversions/into/string.rs

@@ -1,3 +1,5 @@
+use std::sync::Arc;
+
 use nu_cmd_base::input_handler::{operate, CmdArgument};
 use nu_engine::command_prelude::*;
 use nu_protocol::{into_code, Config};
@@ -7,7 +9,7 @@ use num_format::ToFormattedString;
 struct Arguments {
     decimals_value: Option<i64>,
     cell_paths: Option<Vec<CellPath>>,
-    config: Config,
+    config: Arc<Config>,
 }
 
 impl CmdArgument for Arguments {
@@ -174,7 +176,7 @@ fn string_helper(
             })
         }
     } else {
-        let config = engine_state.get_config().clone();
+        let config = stack.get_config(engine_state);
         let args = Arguments {
             decimals_value,
             cell_paths,