[iOS] Hardware keyboard text editing shortcuts

[moffatman]

Implementing these methods enables the following keyboard shortcuts:

• Command-A to select all
• Command-X to cut
• Command-C to copy
• Command-V to paste

I am not completely sure about the delete method, but it's another part of the specification that could be easily implemented. I think it will likely enable Edit->Delete when running Flutter iOS on Apple Silicon Macs, but I don't have one to test it on.

I could not find a way to get the Command-Left / Command-Right shortcuts for Beginning of line / End of line to work.

Should fix flutter/flutter#73950, flutter/flutter#70104, and flutter/flutter#68514.

Pre-launch Checklist

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• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides].
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See [testing the engine] for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
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[dkwingsmt]

The code looks ok, but I don't quite get why this change solves all three issues. Can you give a brief explanation?

[moffatman]

The code looks ok, but I don't quite get why this change solves all three issues. Can you give a brief explanation?

UIKit checks if the current responder can handle these selectors in order to enable the text editing shortcuts for cut/copy/paste. This includes the command-c, command-v... keyboard shortcuts, as well as the buttons above the software keyboard. It's not possible to test for that functionality, since XCTest has no support to trigger keyboard shortcuts, it can only type normal text. But I found after this change that the hardware keyboard editing shortcuts work.

[dkwingsmt]

Thanks for the explanation. Go ahead and merge when you feel like.