Added help externs command (#8403)

# Description

`help externs` - command, which list external commands

Closes https://github.com/nushell/nushell/issues/8301

# User-Facing Changes

```nu
$ help externs
╭───┬──────────────┬─────────────┬───────────────────────────────────────────────────╮
│ # │     name     │ module_name │                       usage                       │
├───┼──────────────┼─────────────┼───────────────────────────────────────────────────┤
│ 0 │ git push     │ completions │ Push changes                                      │
│   │              │             │                                                   │
│ 1 │ git fetch    │ completions │ Download objects and refs from another repository │
│   │              │             │                                                   │
│ 2 │ git checkout │ completions │ Check out git branches and files                  │
│   │              │             │                                                   │
╰───┴──────────────┴─────────────┴───────────────────────────────────────────────────╯
```

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-engine/src/scope.rs

@@ -463,6 +463,52 @@ impl<'e, 's> ScopeData<'e, 's> {
         sig_records
     }
 
+    pub fn collect_externs(&self, span: Span) -> Vec<Value> {
+        let mut externals = vec![];
+        for ((command_name, _), decl_id) in &self.commands_map {
+            let decl = self.engine_state.get_decl(**decl_id);
+
+            if decl.is_known_external() {
+                let mut cols = vec![];
+                let mut vals = vec![];
+
+                let mut module_commands = vec![];
+                for module in &self.modules_map {
+                    let module_name = String::from_utf8_lossy(module.0).to_string();
+                    let module_id = self.engine_state.find_module(module.0, &[]);
+                    if let Some(module_id) = module_id {
+                        let module = self.engine_state.get_module(module_id);
+                        if module.has_decl(command_name) {
+                            module_commands.push(module_name);
+                        }
+                    }
+                }
+
+                cols.push("name".into());
+                vals.push(Value::String {
+                    val: String::from_utf8_lossy(command_name).to_string(),
+                    span,
+                });
+
+                cols.push("module_name".into());
+                vals.push(Value::String {
+                    val: module_commands.join(", "),
+                    span,
+                });
+
+                cols.push("usage".to_string());
+                vals.push(Value::String {
+                    val: decl.usage().into(),
+                    span,
+                });
+
+                externals.push(Value::Record { cols, vals, span })
+            }
+        }
+
+        externals
+    }
+
     pub fn collect_aliases(&self, span: Span) -> Vec<Value> {
         let mut aliases = vec![];
         for (alias_name, alias_id) in &self.aliases_map {