Add @babel/plugin-transform-nullish-coalescing-operator and @rollup/p…

[loycoder]

hello， Fixed this problem： https://github.com/sampotts/plyr/issues/2782, please help review the code, thank you

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[iamvignesht]

@sampotts, Could you please merge and release new npm verison?

[sampotts]

Thanks, I'll get an updated version on npm asap.