Which supports all the existing actions: resize, crop, fit, fill.
But it also allows plain format conversions:
```
{{ $img = $img.Process "webp" }}
```
Which will be a simple re-encoding of the source image.
Fixes#11483
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
