Add support for thumbnail alt text

[blake-mealey]

What type of feature request is this?

• [x] Mantle doesn't support a Roblox feature I would like to use
• [ ] Mantle CLI improvement
• [ ] Other

Describe your problem

New feature: https://devforum.roblox.com/t/alt-text-for-experience-thumbnails/2066564

Describe the solution you'd like

Add an option for thumbnail alt text