HTML export option

[badlogic]

Provide a new export option that creates a .html file with assets embedded as data URIs that uses the spine-ts web player to display the skeleton. The exported .html file will reference the correct runtime from our servers by default, but also offer a checkbox option to embed the runtime itself in the .html file to be fully self-contained.

In the runtime embedding case, we need to decide between these two options:

1. The runtime is shipped with the editor, requiring a new editor release every time we bug fix the runtime.
2. The runtime is fetched from GitHub/our servers on demand, which introduces a potential error case and will not work when offline.

[Dongyifengs]

Can we join the plan for 4.3?

[Dongyifengs]

@NathanSweet @badlogic

[badlogic]

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