[low priority refactoring] generalize executing JS during scraping

If doc sites use client-side JavaScript to produce or fetch some of their content, we need to launch a headless browser and execute it. Currently scraper subclasses do that individually using capybara, e.g.
https://github.com/freeCodeCamp/devdocs/blob/master/lib/docs/scrapers/angularjs.rb#L42-L45
but it might be better to extract a helper method or config option, a little more like this:
https://github.com/freeCodeCamp/devdocs/blob/master/lib/docs/scrapers/webpack.rb#L43-L46

cc @dangeranger

首先比较 lib/docs/scrapers/angularjs.rb#L42-L45 与 lib/docs/scrapers/webpack.rb#L43-L46，以了解现有的基于 Capybara 的 JavaScript 执行模式。跟踪 scraper 配置和辅助工具的组织方式，然后定义一种能够保留相关 scraper 行为的共享方案，并验证受影响的 scraper 仍会生成预期内容。

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