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

testscripts/commands/env.txt

@@ -0,0 +1,5 @@
+# Test the hugo env command.
+
+hugo env
+stdout 'GOARCH'
+! stderr .