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

source/fileInfo.go

@@ -96,6 +96,7 @@ type FileWithoutOverlap interface {
 	// Hugo content files being one of them, considered to be unique.
 	UniqueID() string
 
+	// For internal use only.
 	FileInfo() hugofs.FileMetaInfo
 }
 
@@ -182,6 +183,7 @@ func (fi *FileInfo) UniqueID() string {
 }
 
 // FileInfo returns a file's underlying os.FileInfo.
+// For internal use only.
 func (fi *FileInfo) FileInfo() hugofs.FileMetaInfo { return fi.fi }
 
 func (fi *FileInfo) String() string { return fi.BaseFileName() }