Always populate config record during startup (#14435)

# Description

As a bit of a follow-on to #13802 and #14249, this (pretty much a
"one-line" change) really does *always* populate the `$env.config`
record with the `nu-protocol::config` defaults during startup. This
means that an `$env.config` record is value (with defaults) even during:

* `nu -n` to suppress loading of config files
* `nu -c <commandstring>`
* `nu <script>`

# User-Facing Changes

There should be no case in which there isn't a valid `$env.config`.

* Before:

  ```nushell
  nu -c "$env.config"
  # -> Error
  ```

* After:

  ```nushell
  nu -c "$env.config"
  # -> Default $env.config record
  ```

Startup time impact is negligible (17.072µs from `perf!` on my system) -
Seems well worth it.

# Tests + Formatting

Added tests for several `-n -c` cases.

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting

Config chapter update still in progress.

tests/repl/test_config_path.rs

@@ -298,3 +298,81 @@ fn test_xdg_config_symlink() {
         );
     });
 }
+
+#[test]
+fn no_config_does_not_load_env_files() {
+    let nu = nu_test_support::fs::executable_path().display().to_string();
+    let cmd = format!(
+        r#"
+            {nu} -n -c "view files | where filename =~ 'env\\.nu$' | length"
+        "#
+    );
+    let actual = nu!(cmd);
+
+    assert_eq!(actual.out, "0");
+}
+
+#[test]
+fn no_config_does_not_load_config_files() {
+    let nu = nu_test_support::fs::executable_path().display().to_string();
+    let cmd = format!(
+        r#"
+            {nu} -n -c "view files | where filename =~ 'config\\.nu$' | length"
+        "#
+    );
+    let actual = nu!(cmd);
+
+    assert_eq!(actual.out, "0");
+}
+
+#[test]
+fn commandstring_does_not_load_config_files() {
+    let nu = nu_test_support::fs::executable_path().display().to_string();
+    let cmd = format!(
+        r#"
+            {nu} -c "view files | where filename =~ 'config\\.nu$' | length"
+        "#
+    );
+    let actual = nu!(cmd);
+
+    assert_eq!(actual.out, "0");
+}
+
+#[test]
+fn commandstring_does_not_load_user_env() {
+    let nu = nu_test_support::fs::executable_path().display().to_string();
+    let cmd = format!(
+        r#"
+            {nu} -c "view files | where filename =~ '[^_]env\\.nu$' | length"
+        "#
+    );
+    let actual = nu!(cmd);
+
+    assert_eq!(actual.out, "0");
+}
+
+#[test]
+fn commandstring_loads_default_env() {
+    let nu = nu_test_support::fs::executable_path().display().to_string();
+    let cmd = format!(
+        r#"
+            {nu} -c "view files | where filename =~ 'default_env\\.nu$' | length"
+        "#
+    );
+    let actual = nu!(cmd);
+
+    assert_eq!(actual.out, "1");
+}
+
+#[test]
+fn commandstring_populates_config_record() {
+    let nu = nu_test_support::fs::executable_path().display().to_string();
+    let cmd = format!(
+        r#"
+            {nu} --no-std-lib -n -c "$env.config.show_banner"
+        "#
+    );
+    let actual = nu!(cmd);
+
+    assert_eq!(actual.out, "true");
+}