Small simplifications (redo)

NOTE: This is a redo of #3243, after I did something stupid in Git and messed up the branch history. (😮!)

These are an assortment of small tweaks.

My goal was to make these bits of code easier to understand at a glance. There’s plenty of subjectivity in that, to be sure.

Some of these make use of Javascript features that were not available a few years ago. (Both syntax and APIs of built-in objects.) Hopefully, I applied them in a way that makes it easier to understand each block of code. Since Eleventy v3 is still in alpha, I thought this would be a good time to introduce them, since they are supported pretty much everywhere and a major version increment doesn’t have to worry about breaking compatibility.