Update dependency html-minifier to v4

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change	References
html-minifier	devDependencies	major	^3.5.16 -> ^4.0.0	homepage, source

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Release Notes

kangax/html-minifier

### [`v4.0.0`](https://togithub.com/kangax/html-minifier/releases/v4.0.0) [Compare Source](https://togithub.com/kangax/html-minifier/compare/v3.5.21...v4.0.0) ##### Bug fixes - handle custom fragments within CSS/JS correctly ([#​1001](https://togithub.com/kangax/html-minifier/issues/1001), [#​1015](https://togithub.com/kangax/html-minifier/issues/1015)) ##### Changes - Drop Node.js < 6 support ##### Improvements - implement `continueOnParseError` to treat invalid characters as text ([#​1004](https://togithub.com/kangax/html-minifier/issues/1004)) - minify Content-Security-Policy ([#​947](https://togithub.com/kangax/html-minifier/issues/947), [#​975](https://togithub.com/kangax/html-minifier/issues/975), [#​1014](https://togithub.com/kangax/html-minifier/issues/1014)) - upgrade to `commander 2.19.0` - upgrade to `grunt-contrib-uglify 4.0.1` - upgrade to `gruntify-eslint 5.0.0` - upgrade to `uglify-js 3.5.1`

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