185: Implement UMD support

[KaannKara]

Summary

• Add a Rollup config for the viewer package so the compiled ESM entry is also emitted as a UMD bundle.
• Expose the UMD bundle path through unpkg and jsdelivr package fields.
• Keep three external in the UMD build and map it to the global THREE.

Closes #185

Validation

• corepack yarn install --ignore-scripts
• corepack yarn workspace web-ifc-viewer build
• corepack yarn build
• git diff --check

Note

• corepack yarn workspace web-ifc-viewer test currently fails before running tests because Jest 27 attempts to require viewer/jest.config.ts while this package is configured with type: module on the local Node runtime.