Add optimization tag to proposals

[me2beats]

For example this proposal can have this tag
#5351

As well as occlusion, lods etc.

Optimization is one of most important parts of game development

[Calinou]

There are usability and performance labels on the main Godot repository. I suppose these could also be added to the godot-proposals repository.

[Calinou]

performance and usability labels were added to godot-proposals issues where relevant.

[me2beats]

Yay cool!