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
This is how it may look like with a extended build:
```
hugo v0.107.0-6445b1e9ff963b07c55d9d69cb9abef8ef21fc5d+extended darwin/arm64 BuildDate=2022-12-06T11:21:50Z
GOOS="darwin"
GOARCH="arm64"
GOVERSION="go1.19.3"
github.com/sass/libsass="3.6.5"
github.com/webmproject/libwebp="v1.2.4"
github.com/sass/dart-sass-embedded/protocol="1.1.0"
github.com/sass/dart-sass-embedded/compiler="1.56.1"
github.com/sass/dart-sass-embedded/implementation="1.56.1"
```