feat: make global flags settable

[mdonnalley]

Makes it so that global flags are settable. This allows users to have a base command class that defines flags that should be inherited by all child instances.

export abstract class MyBaseCommand extends Command {
  public static globalFlags = {
    interactive: Flags.boolean({
      description: 'Interactive Command Completion',
      default: false,
      helpGroup: 'GLOBAL',
    }),
  };
}

This was possible before but it required a lot of unpleasant code:

export abstract class MyBaseCommand extends Command {
  private static specialFlags = {
    interactive: Flags.boolean({
      description: 'Interactive Command Completion',
      default: false,
      helpGroup: 'GLOBAL',
    }),
  };

  // eslint-disable-next-line @typescript-eslint/no-explicit-any
  public static get flags(): Interfaces.FlagInput<any> {
    // @ts-expect-error because private member
    // eslint-disable-next-line @typescript-eslint/no-explicit-any
    return this._flags as Interfaces.FlagInput<any>;
  }

  // eslint-disable-next-line @typescript-eslint/no-explicit-any
  public static set flags(flags: Interfaces.FlagInput<any>) {
    // @ts-expect-error because private member
    // eslint-disable-next-line @typescript-eslint/no-explicit-any
    this._flags = Object.assign({}, super.globalFlags, MyBaseCommand.specialFlags, flags) as Interfaces.FlagInput<any>;
  }
}