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

lib/micromark-parse.mjs

@@ -212,7 +212,9 @@ function parseInternal(
   const events = getEvents(markdown, micromarkParseOptions);
 
   // Create Token objects
+  /** @type {MicromarkToken[]} */
   const document = [];
+  /** @type {MicromarkToken[]} */
   let flatTokens = [];
   /** @type {MicromarkToken} */
   const root = {
@@ -282,6 +284,7 @@ function parseInternal(
         );
         current.children = tokens;
         // Avoid stack overflow of Array.push(...spread)
+        // @ts-ignore
         // eslint-disable-next-line unicorn/prefer-spread
         flatTokens = flatTokens.concat(tokens[flatTokensSymbol]);
       }