add static all method

[jlnrrg]

As it looks to me the package is auto generated based on the MDI font. (Or that is how i've done it with the simple icons package)

Please consider adding a static list, where one can get all the icons at one. Example:

static const List<MdiIconData> all = [
    Mdi.abTesting,
  ];

Every user of the library could implement this themself, but with so many icons this would take a lot of time. Under the premise that the mdi.dart is autogenerated, this might be just one line of code for you :pray: .