Added worker export to core and core-mt package.json

[Montaire]

This allows the ESM based bundlers to resolve and bundle the packages properly. Without this it's hard to include the dynamically loaded internal javascript files.

Fixes @ffmpeg/core and @ffmpeg/core-mt missing export for the ffmpeg-core.worker.js file.

758

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[dumbmoron]

@ffmpeg/core does not have a ffmpeg-core.worker.js file btw