Fallback for config.buffer_editor from EDITOR (#5614)

For the reedline `buffer_editor` use the `EDITOR` and `VISUAL`
environment variables as fallback.

Same resolution order as #5607

Closes #5430

crates/nu-cli/src/repl.rs

@@ -159,7 +159,26 @@ pub fn evaluate_repl(
             }
         };
 
-        line_editor = line_editor.with_buffer_editor(config.buffer_editor.clone(), "nu".into());
+        let buffer_editor = if !config.buffer_editor.is_empty() {
+            Some(config.buffer_editor.clone())
+        } else {
+            stack
+                .get_env_var(engine_state, "EDITOR")
+                .map(|v| v.as_string().unwrap_or_default())
+                .filter(|v| !v.is_empty())
+                .or_else(|| {
+                    stack
+                        .get_env_var(engine_state, "VISUAL")
+                        .map(|v| v.as_string().unwrap_or_default())
+                        .filter(|v| !v.is_empty())
+                })
+        };
+
+        line_editor = if let Some(buffer_editor) = buffer_editor {
+            line_editor.with_buffer_editor(buffer_editor, "nu".into())
+        } else {
+            line_editor
+        };
 
         if config.sync_history_on_enter {
             if is_perf_true {

docs/sample_config/default_config.nu

@@ -190,7 +190,7 @@ let-env config = {
   completion_algorithm: "prefix"  # prefix, fuzzy
   animate_prompt: false # redraw the prompt every second
   float_precision: 2
-  buffer_editor: "emacs" # command that will be used to edit the current line buffer with ctrl+o
+  # buffer_editor: "emacs" # command that will be used to edit the current line buffer with ctrl+o, if unset fallback to $env.EDITOR and $env.VISUAL
   use_ansi_coloring: true
   filesize_format: "auto" # b, kb, kib, mb, mib, gb, gib, tb, tib, pb, pib, eb, eib, zb, zib, auto
   edit_mode: emacs # emacs, vi