Bring back SizeSlug option to doctl compute droplet list

**What is the problem this feature would solve? Please describe.**
There is no easy way to know *exactly* what droplet I'm using - neither from the UI nor CLI. As far as I researched the only way is to use JSON output in `doctl compute droplet list -o json`.

**Describe the solution you'd like**
Let's add SizeSlug in as a column in `doctl compute droplet list`

**Additional context**
I saw a few bug reports that it's been there, but I didn't find a way to show the exact droplet slug.