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

lib/md054.mjs

@@ -5,9 +5,9 @@ import { getDescendantsByType } from "../helpers/micromark-helpers.cjs";
 import { getReferenceLinkImageData, filterByTypesCached } from "./cache.mjs";
 
 const backslashEscapeRe = /\\([!"#$%&'()*+,\-./:;<=>?@[\\\]^_`{|}~])/g;
-const removeBackslashEscapes = (text) => text.replace(backslashEscapeRe, "$1");
+const removeBackslashEscapes = (/** @type {string} **/ text) => text.replace(backslashEscapeRe, "$1");
 const autolinkDisallowedRe = /[ <>]/;
-const autolinkAble = (destination) => {
+const autolinkAble = (/** @type {string} */ destination) => {
   try {
     // eslint-disable-next-line no-new
     new URL(destination);
@@ -73,9 +73,11 @@ export default {
           const referenceString = getDescendantsByType(link, [ "reference", "referenceString" ])[0]?.text;
           const isCollapsed = (referenceString === undefined);
           const definition = definitions.get(referenceString || label);
-          destination = definition && definition[1];
-          isError = destination &&
-            (isShortcut ? !shortcut : (isCollapsed ? !collapsed : !full));
+          destination = (definition && definition[1]) || "";
+          isError = Boolean(
+            destination &&
+            (isShortcut ? !shortcut : (isCollapsed ? !collapsed : !full))
+          );
         }
       }
       if (isError) {