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

lib/markdownit.cjs

@@ -127,6 +127,7 @@ function annotateAndFreezeTokens(tokens, lines) {
     }
     // Annotate children with lineNumber
     if (token.children) {
+      /** @type {number[]} */
       const codeSpanExtraLines = [];
       if (token.children.some((child) => child.type === "code_inline")) {
         forEachInlineCodeSpan(token.content, (code) => {
@@ -140,7 +141,7 @@ function annotateAndFreezeTokens(tokens, lines) {
         if ((child.type === "softbreak") || (child.type === "hardbreak")) {
           lineNumber++;
         } else if (child.type === "code_inline") {
-          lineNumber += codeSpanExtraLines.shift();
+          lineNumber += codeSpanExtraLines.shift() || 0;
         }
       }
     }