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 `glob` was outdated, and it's easier to just toss it than to upgrade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 It was super outdated.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

@@ -0,0 +1,72 @@
+"use strict"
+
+const {tryFetch} = require("./try-fetch.js")
+
+function checkKnownCorrectRequestFail(href, headers, status, body) {
+	if (status >= 400 && status <= 499) {
+		return `${href} is a broken link (status: ${status})`
+	}
+
+	// Don't fail if something weird shows up - it's the internet. Just log it and move on.
+	// However, some more sophisticated logging is useful.
+	let message = `HTTP error for ${href} (status: ${status})`
+
+	for (const [name, value] of Object.entries(headers)) {
+		if (Array.isArray(value)) {
+			for (const v of value) message = `${message}\n>${name}: ${v}`
+		} else {
+			message = `${message}\n>${name}: ${value}`
+		}
+	}
+
+	if (body !== "") {
+		message = `${message}\n>${body}`
+	}
+
+	return message
+}
+
+/**
+ * Returns `undefined` if no error, a string if an error does occur.
+ * @param {(message?: string)} callback
+ */
+function checkHttp(href, callback) {
+	// Prefer https: > http: where possible, but allow http: when https: is inaccessible.
+
+	const url = new URL(href)
+	url.hash = ""
+
+	const isHTTPS = url.protocol === "https:"
+	url.protocol = "https:"
+
+	tryFetch(url, (headers, status, body, sslError) => {
+		if (status >= 200 && status <= 299) {
+			if (isHTTPS) {
+				return callback()
+			} else {
+				return callback(`Change ${href} to use \`https:\``)
+			}
+		}
+
+		if (!sslError) {
+			return callback(checkKnownCorrectRequestFail(href, headers, status, body))
+		}
+
+		url.protocol = "http:"
+		tryFetch(url, (headers, status, body) => {
+			if (status >= 200 && status <= 299) {
+				if (isHTTPS) {
+					return callback(`Change ${href} to use \`http:\``)
+				} else {
+					return callback()
+				}
+			}
+
+			return callback(checkKnownCorrectRequestFail(href, headers, status, body))
+		})
+	})
+}
+
+module.exports = {
+	checkHttp,
+}