Add support for data url loading

[ABrozovic]

Enables users to pass dataURLs directly in the spineAtlasFile and image Metadata fields. This enhancement expands the flexibility and convenience of working with spine animations by allowing dataURLs as valid inputs.

[miltoncandelero]

You have some things mixed 😬

the atlas file (.atlas) should be text/plain.

application/octet-stream should be for binary skeleton files .skel

[ABrozovic]

I'm using FileReader's readAsDataURL method which assigns the MIME type "application/octet-stream" to the resulting data URL when reading a plain text file.
But you are correct, const validAtlasMIME = ['application/octet-stream', 'text/plain'];. would be a more accurate evaluation.