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

markup/tableofcontents/tableofcontents.go

@@ -237,6 +237,7 @@ var DefaultConfig = Config{
 type Config struct {
 	// Heading start level to include in the table of contents, starting
 	// at h1 (inclusive).
+	// <docsmeta>{ "identifiers": ["h1"] }</docsmeta>
 	StartLevel int
 
 	// Heading end level, inclusive, to include in the table of contents.