Use destructuring to access createApp, etc. in render-function.md

[skirtles-code]

This is part of #773. The objective is to make more of the examples agnostic to whether they're using a CDN or ESM build. render-function.md is one of the pages where the Vue. prefix is most conspicuous so I've started there.

The first example still includes the global Vue but it is destructured on the first line to resemble an import. Most of the other examples are code fragments so that line is omitted. I have included it whenever a new global function is introduced, to make it clear where it comes from.

Many of the examples just show an isolated render function, which makes it tricky to also show the destructuring. I wasn't sure how best to show that, so I included a commented ellipsis. It's a little clunky but it was the best idea I had.

I haven't explicitly acknowledged the difference between the builds on this page. That might be added later but I'm unclear precisely how I'm going to approach that on other pages yet.