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
Primary motivation is documentation, but it will also hopefully simplify the code.
Also,
* Lower case the default output format names; this is in line with the custom ones (map keys) and how
it's treated all the places. This avoids doing `stringds.EqualFold` everywhere.
Closes#10896Closes#10620
