Reconsider lang attribute

**Is your feature request related to a problem? Please describe.**
Lighthouse audits recommend adding a lang attribute to your HTML. With the goal of being minimal / zero config, it would be nice to set that attribute without writing a custom template or plugin.

**Describe the solution you'd like**
A solution similar to #562

**Describe alternatives you've considered**
Writing a custom template or plugin for it. The suggested solution of HTML Webpack Template has not been update in years and seems to have issues with Webpack 4?

**Additional context**
I can see requesting arbitrary extensions could lead to feature bloat. I do think it's reasonable to use Lighthouse recommendations as the boundary to prevent feature bloat.

Start by reading the issue's reference to #562 and reviewing how html-webpack-plugin handles configuration and generated HTML. Consider how a lang attribute could be configured without a custom template or plugin, while avoiding arbitrary extension points. Done means a documented, tested way to set the attribute that works with the current webpack 4-compatible setup.

Written by the indexing model from the issue text.