<!--- Provide a general summary of your changes in the title above -->
## Types of changes
<!--- What types of changes does your code introduce? Put an `x` in all the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing functionality to change)
## Description
Webpack requires `uglifyjs-webpack-plugin@1.x`. thus `uglifyjs-webpack-plugin@2.x` may not resolve correctly. Also, the webpack team decided to go with `terser-webpack-plugin` (https://github.com/webpack/webpack/pull/8036).
## Checklist:
<!--- Put an `x` in all the boxes that apply. -->
<!--- If your change requires a documentation PR, please link it appropriately -->
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- [ ] My change requires a change to the documentation.
- [ ] I have updated the documentation accordingly. (PR: #)
- [ ] I have added tests to cover my changes (if not applicable, please state why)
- [ ] All new and existing tests passed.
