fix(scripts): detect version heading at file start

indexOf('\n## ') required a preceding newline, so a root
CHANGELOG that begins directly with a "## <version>" heading
(e.g. after a human strips the "# Changelog" header) was
treated as having no version block, and new content got
appended to the end instead of spliced at the top. Switch to
a multiline regex that matches "## " at any line start,
including line 0.

Author: burczu

scripts/consolidate-changelog.ts

@@ -171,10 +171,10 @@ function consolidate(): void {
 
   if (fs.existsSync(ROOT_CHANGELOG)) {
     const content = fs.readFileSync(ROOT_CHANGELOG, 'utf-8');
-    const firstVersionIdx = content.indexOf('\n## ');
-    if (firstVersionIdx !== -1) {
-      header = content.slice(0, firstVersionIdx + 1);
-      body = content.slice(firstVersionIdx + 1);
+    const firstHeadingMatch = content.match(/^## /m);
+    if (firstHeadingMatch && firstHeadingMatch.index !== undefined) {
+      header = content.slice(0, firstHeadingMatch.index);
+      body = content.slice(firstHeadingMatch.index);
     } else {
       header = content.endsWith('\n') ? content : content + '\n';
       body = '';