Add Ipad 12 Gen 2 and 4

[caseycrogers]

This PR adds the 12.9 inch gen 2 and 4 Ipad Pro models. These devices are important because screenshots for them are required by the App Store to publish an app (a 5.5 inch Iphone screenshot is also required and missing from device_frame, but the Iphone SE is a decent stand-in for it): https://help.apple.com/app-store-connect/#/devd274dd925

I could not edit your official figma document, so I added the new designs in a copy of it: https://www.figma.com/file/guISu5DWSmx9bzUpAR0tu7/DevicePreview---Frames-(Copy)?node-id=0%3A1

If you give me edit access to the official document (caseycrogers@berkeley.edu) I can copy them over. Otherwise, you could copy them in yourself.

The two new frames are designed in figma relative to a reference image-they were not taken from the community devices figma design like the other frames. Each of the screen objects are the exact aspect ratio of the real screen. Media query related values were gotten by running emulators of the respective devices and copying the results of MediaQuery.of....

I left version bumping and updating the changelog to you...

[caseycrogers]

cc @aloisdeniel

[aloisdeniel]

Sorry for being so long to answer!

Thanks a lot for your contribution!

[Musta-Pollo]

Hi, awesome work! Thank you so much!. I would just consider adding both of those devices to the get all in device_frame/lib/src/devices/ios/devices.dart

So other people can easily took advantage of them.

// All devices.
  List<DeviceInfo> get all => [
        // Phones
        iPhone12Mini,
        iPhone12,
        iPhone12ProMax,
        iPhone13Mini,
        iPhone13,
        iPhone13ProMax,
        iPhoneSE,
        //Tablets
        iPadAir4,
        iPad,
        iPadPro11Inches,
       //Extra IPads
       iPad12InchesGen2,
       iPad12InchesGen4
      ];```