Support for ES6 minification

Hi

the html-webpack-plugin is making heavy use of html-minifier to keep the html compact.

Now that many browsers support ES6 out of the box, people start using ES6 inside script tags.
According to @evilebottnawi from the webpack team using ES6 is not possible because html-minifier is not capable of parsing ES6: https://github.com/jantimon/html-webpack-plugin/issues/1262

When webpack was faced with the same problem they made a switch to terser which has now about 6 million downloads per week.

uglify-es is no longer maintained and uglify-js does not support ES6+.

terser is a fork of uglify-es that mostly retains API and CLI compatibility with uglify-es and uglify-js@3.

(taken from their official readme)

There was already a proposal to do the same change for the html-minifier: #1034

It would mean to replace https://github.com/kangax/html-minifier/blob/51ce10f4daedb1de483ffbcccecc41be1c873da2/package.json#L63 with terser

[willfarrell]

Workaround:

const jsMinify = require('terser').minify;
const htmlMinify = require('html-minifier').minify;
const htmlMinifyOptions = {
  ...
  minifyJS: (text, inline) => {
        const res = jsMinify(text, { warnings: true })
        if (res.warnings) console.log(res.warnings)
        if (res.error) {
            console.log(text)
            throw res.error
        }
        return res.code
    }
...
}
...
output = htmlMinify(output, htmlMinifyOptions)

[DanielRuf]

html-minifier-terser v5.0.0 is released now: https://www.npmjs.com/package/html-minifier-terser