fix ESM dist target

[FezVrasta]

fixes #40

[jourdain]

@xhlulu what do you think? This is going to affect how dash-vtk is building against it. It should not be too much trouble to update it, but I wanted to have your thought on it. So far we were using the UMD build, but with that change only cjs and esm will be available.

@FezVrasta, we were using the UMD in dash-vtk to get a single file dependency. But we might be able to use cjs instead.

[xhluca]

I'm not familiar with JS bundling, so I'll let Alex J decide here :)

[FezVrasta]

I can add the UMD bundle back as a separate Rollup build if the existing UMD is required

[FezVrasta]

I restored the UMD target.

[jourdain]

🎉 This PR is included in version 1.8.5 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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