Add ability to define reusable keyframes in `theme`

[o-alexandrov]

Summary

Related issues

• [core] Errors thrown in non-browser environment (unsafe when referenced) #8
• Support runtime / "non bundler integration" mode? #59

Examples

A user should be able to use pigment.keyframes() API in a non-browser environment, to add generated keyframes names to theme.

Motivation

To not require the user to separately import keyframes/animation.
Reusing theme is easier due to existent exposure of theme variable.

Search keywords: theme, keyframes