chore: update miscellaneous parts of the source to reasonably modern language alternatives, remove polyfills, improve some JSDoc comments, remove traces of IE

BREAKING: In a minority of cases syntax updates may break apps running in very old browsers (such as Internet Explorer), or apps that build Docsify in a custom way with old build tools. To upgrade, build Docsify for older browsers in a custom way, or update existing build tools to handle newer syntax.

DEPRECATED: `$docsify.themeColor` is deprecated and will be eventually removed, use a `--theme-color` CSS variable in your style sheet.