Mangling error fixed with Terser, not UglifyJS

[stuartlangridge]

https://github.com/optoolco/tonic/blob/master/HELP.md under the heading "Webpack 4+ Mangling Error" suggests that a "Mangling" error in the console when using webpack is fixable by adding the keep_fnames option to the UglifyJS config in your webpack config. However, Webpack now comes with TerserPlugin out of the box, and it's that that's mangling the names by default; it may be worth adding to those docs to suggest making a change as follows:

...
    optimization: {
        minimizer: [ // needed for Tonic
            new TerserPlugin({
                terserOptions: {
                    keep_fnames: true
                }
            })
        ]
    }
...

to the webpack configuration. (This is exactly what's there already, except for Terser, not UglifyJS).

This isn't a PR because there may be other better Tonic-ish options that should also be set for Terser and I don't know what they would be.