Fix RAW images file type detection

For Sony and Canon Raw file formats. They are based on the tiff format
and have the same magic number data so they are incorrectly classified
as tiff images.

Defining them as subtypes of tiff solves this issue:

```ruby
Marcel::MimeType.for Pathname.new("/Users/afcapel/Downloads/RAW_SONY_ILCE-7M2.ARW"), name: "/Users/afcapel/Downloads/RAW_SONY_ILCE-7M2.ARW"
=> "image/x-raw-sony"
```

Author: afcapel

lib/marcel/mime_type/definitions.rb

@@ -43,6 +43,9 @@
 Marcel::MimeType.extend "image/heif", magic: [[4, "ftypmif1"]], extensions: %w( heif )
 Marcel::MimeType.extend "image/heic", magic: [[4, "ftypheic"]], extensions: %w( heic )
 
+Marcel::MimeType.extend "image/x-raw-sony", extensions: %w( arw ), parents: "image/tiff"
+Marcel::MimeType.extend "image/x-raw-canon",  extensions: %w( cr2 crw ), parents: "image/tiff"
+
 Marcel::MimeType.extend "video/mp4", magic: [[4, "ftypisom"], [4, "ftypM4V "]], extensions: %w( mp4 m4v )
 
 Marcel::MimeType.extend "audio/flac", magic: [[0, 'fLaC']], extensions: %w( flac ), parents: "audio/x-flac"