Fixes #30725 #85328

• Introduces a new TextInputConnection type that dispatches text input plugin commands in the form of Intents to Flutter text fields, as an alternative to the current TextInputClient approach. This allows text input plugin authors to add new capabilities to Flutter text fields without having to update a static interface (currently all capabilities have to be added to the TextInputClient interface and it's always a breaking change to add a new one). See https://docs.google.com/document/d/1pNM7Ay3NkIBjE0jIY-c41OdtSc9bv_izO7xJ9OrAT5E/edit
• Removes framework-defined macOS shortcuts. The macOS text input plugin will let the system handle shortcuts/keybindings, and convert selector names retrieved in doCommandBySelector: to framework intents, and send the intents via IntentTextInputConnection.

Future Plans

• deprecate TextInputClient and DeltaTextInputClient.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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