backports #1680

* chore: lockfile maintenance

* chore: update memfs to ^3.1.2

Fixes a bug in `mkdir('/')`, which causes tests fail to run in webpack 5

* test: setup webpack 5 test

* fix: fix webpack 5 compatibility and add test for it

* ci: fix yaml grammar

* [RU] Translation moved to VuePress

* custom-block.md example fix

* fix link

* options.md правка

* testing.md fix links

* linting.md fix link

* options.md added new section

* options.md add exposeFilename

* guide/README.md update

* README.md обновление

* docs: [RU] update

* pre-processors.md update

* custom-blocks.md update

* pre-processors.md fix example

Fixes #1639

Allow user to run `npm repo vue-loader` and remove warn running `npm`.

According to the documentation
https://vue-loader.vuejs.org/guide/extract-css.html#webpack-3
and some private feedbacks, there are still many users use
vue-loader v15 along with webpack 3.