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 `glob` was outdated, and it's easier to just toss it than to upgrade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 It was super outdated.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/_lint-docs/task-queue.js

@@ -0,0 +1,35 @@
+"use strict"
+
+// CI needs a much lower limit so it doesn't hang.
+const maxConcurrency = process.env.CI === "true" ? 5 : 20
+
+const queue = []
+let running = 0
+
+function runTask(task, callback) {
+	process.nextTick(task, (...args) => {
+		process.nextTick(callback, ...args)
+		if (running === maxConcurrency) {
+			const [nextTask, nextCallback] = queue.splice(0, 2)
+			runTask(nextTask, nextCallback)
+		}
+	})
+}
+
+/**
+ * @template {any[]} A
+ * @param {(callback: (...args: A) => void) => void} task
+ * @param {(...args: A) => void} callback
+ */
+function submitTask(task, callback) {
+	if (running < maxConcurrency) {
+		running++
+		runTask(task, callback)
+	} else {
+		queue.push(task, callback)
+	}
+}
+
+module.exports = {
+	submitTask,
+}