Add extractCSS option to schema to enable override of extract-css plugin settings

[mediaessenz]

What problem does this feature solve?

Currently it is IMO not possible to override the filename of an extracted css, by defining an extractCSS object in the vue.config.js.
The object itself would be already used here:

vue-cli/packages/@vue/cli-service/lib/config/css.js

Line 106 in e65110f

const userOptions = options.extractCSS && typeof options.extractCSS === 'object'

but is not allowed by the schema, defined in options.js of the parent dir.

What does the proposed API look like?

Add extractCSS: joi.object() to the schema definition inside cli-service/lib/options.js

[mediaessenz]

Meanwhile I found a way (thank to @eliperelman) to do this with help of the chainWebpack inside my vue.config.js:

chainWebpack: config => {
    config
        .when(process.env.NODE_ENV === 'production', plugin => {
            plugin.plugin('extract-css').tap(([options, ...args]) => [
                Object.assign({}, options, { filename: 'Css/[name].css' }),
                ...args
            ])
        })
},

[xxxxxMiss]

@mediaessenz How to use the options.publicPath of extractCss?