Add send: vichangemode to reedline config (#16327)

# Description
Allows custom bindings (non-chord) to send a `edit_mode: vi` mode change
via the new `ReedlineEvent::ViChangeMode`
Takes https://github.com/nushell/reedline/pull/932

# User-Facing Changes
You can now set bindings which change the Vi mode. (This still has the
same rules for defining the key-combination: Only modifiers and single
keys are supported)
To do so send a `vichangemode` event with the `mode` field to set
`normal`, `insert`, or `visual`

```nushell
$env.config.keybindings ++=
	[{
	    name: modechangetest
	    modifier: control
	    keycode: "char_["
	    mode: [vi_normal, vi_insert]
	    event: {send: vichangemode, mode: normal}
	}]

```

crates/nu-cli/src/reedline_config.rs

@@ -1047,6 +1047,10 @@ fn event_from_record(
             ReedlineEvent::ExecuteHostCommand(cmd.to_expanded_string("", config))
         }
         "openeditor" => ReedlineEvent::OpenEditor,
+        "vichangemode" => {
+            let mode = extract_value("mode", record, span)?;
+            ReedlineEvent::ViChangeMode(mode.as_str()?.to_owned())
+        }
         str => {
             return Err(ShellError::InvalidValue {
                 valid: "a reedline event".into(),