Adds SpellCheck Objective C code in engine. This is part of flutter/flutter#34688

The PR can be landed standalone, it does not affect or add features to the current Flutter.

This PR adds a FlutterSpellCheckPlugin to FlutterEngine. This plugin uses a method channel to communicate with dart. The plugin currently has one method: SpellCheck.initiateSpellCheck, which looks for the first misspelled word in the argument String and returns a result map:

{
   @"startIndex": 0,
   @"endIndex" : 5,
   @"suggestions": [@"suggestion1", @"suggestion2"..]
 }

Design doc: https://docs.google.com/document/d/10Lvkz802QpzAGFi3Q9WVTMVUV-SPJyG3_xs8XfxG59Q/edit?usp=sharing

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 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 ///).
• I signed the CLA.
• All existing and new tests are passing.

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