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 `glob` was outdated, and it's easier to just toss it than to upgrade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 It was super outdated.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/do-lint.js

@@ -0,0 +1,162 @@
+"use strict"
+
+const fs = require("fs")
+const path = require("path")
+
+const {submitTask} = require("./task-queue.js")
+const {processOne} = require("./process-file.js")
+const {root, rel, p, warnError} = require("../_utils.js")
+
+const doNotVisit = /[\\/]node_modules(?:$|[\\/])|[\\/]docs[\\/](?:changelog|recent-changes|migration-[^\\/]*)\.md$/
+
+function lintOne(file, callback) {
+	let warnings = 0
+	let errors = 0
+	let files = 0
+
+	let pending = 1
+
+	function settle() {
+		if (--pending === 0) {
+			callback(warnings, errors, files)
+		}
+	}
+
+	function visitNext(file, contents) {
+		if (contents !== undefined) {
+			files++
+			pending++
+			processOne(file, contents, (w, e) => {
+				warnings += w
+				errors += e
+				settle()
+			})
+		} else {
+			pending++
+			submitTask(fs.readdir.bind(null, file), (err, files) => {
+				if (err) {
+					if (err.code !== "ENOTDIR") {
+						warnError(err)
+					}
+				} else {
+					for (const child of files) {
+						const joined = path.join(file, child)
+						if (!doNotVisit.test(joined)) {
+							visit(joined)
+						}
+					}
+				}
+
+				settle()
+			})
+		}
+	}
+
+	function visit(file) {
+		if (file.endsWith(".md")) {
+			pending++
+			submitTask(fs.readFile.bind(null, file, "utf8"), (err, contents) => {
+				// Not found is fine. Just ignore it.
+				if (!err || err.code === "EISDIR") {
+					visitNext(file, err ? undefined : contents)
+				} else if (err.code !== "ENOENT") {
+					warnError(err)
+				}
+				settle()
+			})
+		} else {
+			visitNext(file, undefined)
+		}
+	}
+
+	visit(file)
+}
+
+function lintAll() {
+	lintOne(root, (warnings, errors, files) => {
+		let problems = ""
+
+		if (errors !== 0) {
+			process.exitCode = 1
+			problems = `${problems}\n${errors} error${errors === 1 ? "" : "s"}`
+		}
+
+		if (warnings !== 0) {
+			problems = `${problems}\n${warnings} warning${warnings === 1 ? "" : "s"}`
+		}
+
+		if (problems !== "") {
+			console.error(`${problems} found in the docs\n`)
+			console.error(`Scanned ${files} file${files === 1 ? "" : "s"}\n`)
+		} else {
+			console.log("The docs are in good shape!\n")
+			console.log(`Scanned ${files} file${files === 1 ? "" : "s"}\n`)
+		}
+	})
+}
+
+function lintFile(file, callback) {
+	if (doNotVisit.test(file)) {
+		if (typeof callback === "function") process.nextTick(callback)
+	} else {
+		lintOne(file, (warnings, errors) => {
+			const relativePath = rel(file)
+
+			let problems = ""
+
+			if (errors !== 0) {
+				problems = `${problems}\n${errors} error${errors === 1 ? "" : "s"}`
+			}
+
+			if (warnings !== 0) {
+				problems = `${problems}\n${warnings} warning${warnings === 1 ? "" : "s"}`
+			}
+
+			if (problems !== "") {
+				console.error(`${problems} found in ${relativePath}\n`)
+			}
+
+			callback?.()
+		})
+	}
+}
+
+function lintWatch() {
+	const timers = new Map()
+
+	const handleFileInner = (filename) => {
+		timers.delete(filename)
+		lintFile(p(filename))
+	}
+
+	const handleFile = (filename) => {
+		const timer = timers.get(filename)
+		if (timer !== undefined) {
+			timer.refresh()
+		} else {
+			timers.set(filename, setTimeout(handleFileInner, 400, filename))
+		}
+	}
+
+	let fileBuffer = []
+
+	lintFile(root, () => {
+		for (const file of fileBuffer) {
+			handleFile(file)
+		}
+		fileBuffer = undefined
+	})
+
+	fs.watch(root, {recursive: true}, (_, filename) => {
+		if (fileBuffer !== undefined) {
+			fileBuffer.push(filename)
+		} else {
+			handleFile(filename)
+		}
+	})
+}
+
+module.exports = {
+	lintAll,
+	lintWatch,
+}