Rewrite docs linter

1. I want to set the stage to deal with #2898 properly.
2. `request` was deprecated years ago. Decided that it's better to just
   move to native Node.js APIs in its place.
3. `glob` was outdated, and it's easier to just toss it than to upgrade
   across a major version.
4. I switched to using Marked's "lexer" directly so I'm not fussing
   with the complexity of renderers. This of course necessitated a more
   complex file processor as its "lexer" is really an AST parser.

I also decided to go a few steps further:
- Drop the cache to simplify everything. I might reverse this later,
  but just caching URLs per-page should be enough to prevent the world
  from crashing down.
- Drop some more dependencies, so I don't have to come back to this
  later nearly as quickly.
- Upgrade to a more modern language version in the scripts.
- Update Marked. It was super outdated.
- Add line and column numbers to the warnings. That took quite a bit of
  work, thanks to a missing Marked feature plus a bug in Marked.

scripts/_utils.js

@@ -0,0 +1,26 @@
+"use strict"
+
+const path = require("path")
+
+const root = path.dirname(__dirname)
+const p = (...args) => path.resolve(root, ...args)
+const rel = (file) => path.relative(root, file).replace(/\\/g, "/")
+const noop = () => {}
+
+function warnError(e) {
+	// Don't care about any of these.
+	if ((/^(?:ECONNRESET|ECONNABORT|EPIPE)$/).test(e.code)) {
+		return
+	}
+
+	process.exitCode = 1
+	console.warn(e.stack)
+}
+
+module.exports = {
+	root,
+	p,
+	rel,
+	warnError,
+	noop,
+}