[3108] [Enhancement] Support extractCss in webpack 4

Mini-css-extract-plugin is the successor of currently used extract-text-webpack-plugin for webpack 4.

Using extractCss=true in webpack 4 fails with:

basic chunk optimization/PROJECT_DIR/node_modules/webpack/lib/Chunk.js:465
		throw new Error("Chunk.entrypoints: Use Chunks.addGroup instead");
		^

Error: Chunk.entrypoints: Use Chunks.addGroup instead
    at Chunk.set (/PROJECT_DIR/node_modules/webpack/lib/Chunk.js:465:9)
    at /PROJECT_DIR/node_modules/extract-text-webpack-plugin/dist/index.js:176:40
    at Array.forEach (<anonymous>)
    at Compilation.<anonymous> (/PROJECT_DIR/node_modules/extract-text-webpack-plugin/dist/index.js:171:18)
    at Compilation.applyPluginsAsyncSeries (/PROJECT_DIR/node_modules/tapable/lib/Tapable.js:206:13)
    at Compilation.seal (/PROJECT_DIR/node_modules/nuxt/node_modules/webpack/lib/Compilation.js:602:8)
    at applyPluginsParallel.err (/PROJECT_DIR/node_modules/nuxt/node_modules/webpack/lib/Compiler.js:504:17)
    at /PROJECT_DIR/node_modules/tapable/lib/Tapable.js:289:11
    at _addModuleChain (/PROJECT_DIR/node_modules/nuxt/node_modules/webpack/lib/Compilation.js:505:11)
    at processModuleDependencies.err (/PROJECT_DIR/node_modules/nuxt/node_modules/webpack/lib/Compilation.js:475:14)
    at process._tickCallback (internal/process/next_tick.js:112:11)
error An unexpected error occurred: "Command failed.
Exit code: 1