Create hugo_stats.json if it's mounted but does not exists

A common pattern for Tailwind 3 is to mount that file to get it on the server watch list.

A common pattern is also to add hugo_stats.json to .gitignore.

This has meant that the first time you start the server (no hugo_stats.json), it just doesn't work as expected.

Fixes #11264

hugofs/files/classifier.go

@@ -31,6 +31,8 @@ const (
 	FilenamePackageHugoJSON = "package.hugo.json"
 	// The NPM package file.
 	FilenamePackageJSON = "package.json"
+
+	FilenameHugoStatsJSON = "hugo_stats.json"
 )
 
 var (