Add ContentTypes to config

This is an empty struct for now, but we will most likely expand on that.

```
[contentTypes]
  [contentTypes.'text/markdown']
```

The above means that only Markdown will be considered a content type. E.g. HTML will be treated as plain text.

Fixes #12274

source/fileInfo.go

@@ -132,6 +132,7 @@ func (fi *File) p() *paths.Path {
 	return fi.fim.Meta().PathInfo.Unnormalized()
 }
 
+// Used in tests.
 func NewFileInfoFrom(path, filename string) *File {
 	meta := &hugofs.FileMeta{
 		Filename: filename,