Compile as a umd module

[blurymind]

there's interest to add dragonbones support to gdevelop via the pixijs version.
The main issue now is that gdevelop requires it to be a umd module. Can support for building to a umd module be added please 🙏 🙂

[FlySkyPie]

@blurymind

I've made a package: https://github.com/FlySkyPie/dragonbones-pixi
but it's only support pixijs 4.X