Standardized create() Methods

[olivervogel]

The create() method is typically a static constructor with the same parameters. Some classes deviate from this standard and also accept variable parameters. This should be standardized by ensuring that the create() method always reflects the constructor's parameters. Other parameters should be handled in dedicated static methods.

The following methods need change:

• Intervention\Image\Format::create()
• Intervention\Image\FileExtension::create()
• Intervention\Image\MediaType::create()