[BC] Improve AjaxCommand definition extension

Problem/Motivation

Current AjaxCommand definition is redundant because command key is defined in the Drupal.definedAjaxCommands interface and this key is needed in ajaxCommand type too.

An AjaxCommand definition example:

declare global {
  namespace Drupal {
    interface definedAjaxCommands {
      // Need to write command key twice.
      testCommand: ajaxCommand<'testCommand', {arg1: string, arg2: boolean}>
    }
  }
}

Proposed resolution

Change ajaxCommand type like below.

Before:

export type ajaxCommand<K extends string, P, R = void> = (ajax: Ajax, response: {command: K} & P, status: number) => R;

After:

export type ajaxCommand<P, R = void> = (ajax: Ajax, response: {command: keyof Drupal.definedAjaxCommands} & P, status: number) => R;

The previous AjaxCommand definition example changes into below.

declare global {
  namespace Drupal {
    interface definedAjaxCommands {
      testCommand: ajaxCommand<{arg1: string, arg2: boolean}>
    }
  }
}

Remaining tasks

API changes

Yes.
If this change is committed, existing definition needs to change.