Create a struct with all of Hugo's config options

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 #10896
Closes #10620

watchtestscripts.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+
+trap exit SIGINT
+
+# I use "run tests on save" in my editor.
+# Unfortantly, changes to text files does not trigger this. Hence this workaround.
+while true; do find testscripts -type f -name "*.txt" | entr -pd touch main_test.go; done