Address new TypeScript warnings in core files, improve type definitions.

lib/md042.mjs

@@ -4,6 +4,8 @@ import { addErrorContext } from "../helpers/helpers.cjs";
 import { getDescendantsByType } from "../helpers/micromark-helpers.cjs";
 import { getReferenceLinkImageData, filterByTypesCached } from "./cache.mjs";
 
+/** @typedef {import("markdownlint").MicromarkToken} MicromarkToken */
+
 /** @type {import("markdownlint").Rule} */
 export default {
   "names": [ "MD042", "no-empty-links" ],
@@ -12,9 +14,9 @@ export default {
   "parser": "micromark",
   "function": function MD042(params, onError) {
     const { definitions } = getReferenceLinkImageData();
-    const isReferenceDefinitionHash = (token) => {
+    const isReferenceDefinitionHash = (/** @type {MicromarkToken} */ token) => {
       const definition = definitions.get(token.text.trim());
-      return (definition && (definition[1] === "#"));
+      return Boolean(definition && (definition[1] === "#"));
     };
     const links = filterByTypesCached([ "link" ]);
     for (const link of links) {