Call node-based checks from a node script rather than Python

There's quite a large overhead to starting up a node script. For example, npm run check:orphan-pages takes ~1.4s on my machine, but console.time shows we only spend ~30ms in the main function. Some of that will be transpiling time, which we can't improve, but some it is avoidable overhead of running many scripts.

We could trim quite a few seconds off ./check simply by having a single node script that calls many checks' main functions, rather than having many separate node scripts.

That is, rather than

npm run check:markdown
npm run check:spelling
npm run check:orphan-pages
# etc.

We'd just have one command (name TBD) that has a single script that calls the main functions of each of these scripts:

zxMain(async () => {
  await checkSpelling();
  await checkMarkdown();
  await checkOrphanPages()
  // etc.
});

This should be a relatively simple fix:

• First, remove process.exit from the scripts and have them export their main function rather than executing it. You could maybe have each main function return it's exit code instead.
• For the checks that accept it, pass through the apis argument as an input to their function
• Update ./check and .github/workflows to use this new meta-script

Once that's working, consider using Promise.all to run the checks asynchronously.