Implements ctrl-T to transpose characters on Mac and iOS

Fixes a bug especially with text fields in a ListTile where space and enter didn't work.

Support for Windows home/end keyboard shortcuts

This reverts commit a24d5566.

Keyboard shortcuts near wordwrapped lines could be buggy before this fix.

Support for more desktop keyboard keys/shortcuts.

No more RawKeyboard listeners in RenderEditable, use Shortcuts instead.

Control + A to select all text is now overridable.

Applies #78522 to existing code, by replacing all occurrences of LogicalKeySet to SingleActivator or ShortcutActivator.

Adds support for overridable delete/backspace keyboard shortcuts

This reverts commit 0ba2f6a8.

Support for keyboard backspace/delete shortcuts

Just a docs typo fix.

This reverts commit 75453e6a because it breaks flutter/test/rendering/editable_test.dart

Landing on red to fix the build.

Landing this on red to fix the build.

TBR=@justinmc

Text editing shortcuts involving the arrow keys are no longer handled by RenderEditable's RawKeyboardListener, they use the new Shortcuts setup.  First PR in a plan to port all text editing keyboard handling to shortcuts.