Update dependency uglifyjs-webpack-plugin to v2

Created by: renovate[bot]

This PR contains the following updates:

Package	Type	Update	New value	References	Sourcegraph
uglifyjs-webpack-plugin	devDependencies	major	^2.0.0	source

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Release Notes

webpack-contrib/uglifyjs-webpack-plugin

v2.0.1

Compare Source

Bug Fixes

• decrease minimum required uglify-js version (#​357) (dc9d038)

v2.0.0

Compare Source

Bug Fixes

• default extract comment condition is case insensitivity

Features

• full coverage schema options validation
• enable inline optimization by default

BREAKING CHANGES

• Switch back to uglify-js (uglify-es is abandoned, if you need uglify ES6 code please use terser-webpack-plugin).
• Changed function signature for warningsFilter option (now first argument is warning, second is source).
• Enforces peerDependencies of "webpack": "^4.3.0.
• Enforces engines of "node": ">= 6.9.0 <7.0.0 || >= 8.9.0

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