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

tpl/tplimpl/template_funcs.go

@@ -71,8 +71,10 @@ var (
 )
 
 type templateExecHelper struct {
-	running bool // whether we're in server mode.
-	funcs   map[string]reflect.Value
+	running    bool // whether we're in server mode.
+	site       reflect.Value
+	siteParams reflect.Value
+	funcs      map[string]reflect.Value
 }
 
 func (t *templateExecHelper) GetFunc(ctx context.Context, tmpl texttemplate.Preparer, name string) (fn reflect.Value, firstArg reflect.Value, found bool) {
@@ -111,6 +113,8 @@ func (t *templateExecHelper) GetMapValue(ctx context.Context, tmpl texttemplate.
 	return v, v.IsValid()
 }
 
+var typeParams = reflect.TypeOf(maps.Params{})
+
 func (t *templateExecHelper) GetMethod(ctx context.Context, tmpl texttemplate.Preparer, receiver reflect.Value, name string) (method reflect.Value, firstArg reflect.Value) {
 	if t.running {
 		switch name {
@@ -123,6 +127,13 @@ func (t *templateExecHelper) GetMethod(ctx context.Context, tmpl texttemplate.Pr
 		}
 	}
 
+	if strings.EqualFold(name, "mainsections") && receiver.Type() == typeParams && receiver.Pointer() == t.siteParams.Pointer() {
+		// MOved to site.MainSections in Hugo 0.112.0.
+		receiver = t.site
+		name = "MainSections"
+
+	}
+
 	fn := hreflect.GetMethodByName(receiver, name)
 	if !fn.IsValid() {
 		return zero, zero
@@ -167,8 +178,10 @@ func newTemplateExecuter(d *deps.Deps) (texttemplate.Executer, map[string]reflec
 	}
 
 	exeHelper := &templateExecHelper{
-		running: d.Running,
-		funcs:   funcsv,
+		running:    d.Conf.Running(),
+		funcs:      funcsv,
+		site:       reflect.ValueOf(d.Site),
+		siteParams: reflect.ValueOf(d.Site.Params()),
 	}
 
 	return texttemplate.NewExecuter(