Add ability to send macOS-specific modifier keys (Command, Option) to `boot_command`

#### Description

I'm using Packer to spin up macOS VMs with [Tart's Packer plugin](https://github.com/cirruslabs/packer-plugin-tart/). It works great, but one quality of life improvement I'd love to see is the ability to use correct references to macOS-specific modifier keys (Command and Option keys) in the `boot_command`.

Specifically, what I'd like to have is:

1. The ability to use `` and its left/right off/on variants to send the Command key.
2. The ability to use `` and its left/right off/on variants to send the Option key.

Right now, `` an `` both map to left and right Command respectively, so I assume these could just be aliased somehow.

I haven't figured out how to send an Option key command yet though, so I'm not sure how that would be implemented.

I'm willing to do this work myself if someone could first give me some guidance or ideas on where and how this modification might be made.