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

hugofs/noop_fs.go

@@ -34,7 +34,7 @@ type noOpFs struct {
 }
 
 func (fs noOpFs) Create(name string) (afero.File, error) {
-	return nil, errNoOp
+	panic(errNoOp)
 }
 
 func (fs noOpFs) Mkdir(name string, perm os.FileMode) error {
@@ -62,7 +62,7 @@ func (fs noOpFs) RemoveAll(path string) error {
 }
 
 func (fs noOpFs) Rename(oldname string, newname string) error {
-	return errNoOp
+	panic(errNoOp)
 }
 
 func (fs noOpFs) Stat(name string) (os.FileInfo, error) {
@@ -74,13 +74,13 @@ func (fs noOpFs) Name() string {
 }
 
 func (fs noOpFs) Chmod(name string, mode os.FileMode) error {
-	return errNoOp
+	panic(errNoOp)
 }
 
 func (fs noOpFs) Chtimes(name string, atime time.Time, mtime time.Time) error {
-	return errNoOp
+	panic(errNoOp)
 }
 
 func (fs *noOpFs) Chown(name string, uid int, gid int) error {
-	return errNoOp
+	panic(errNoOp)
 }