This PR fixes flutter/flutter#76736 by making all 8 (previously 4) modifier keys as critical keys. Critical keys are keys that Flutter checks true state of upon every event and synthesizes events for to make up desynchronized states.

This PR adds a new type of KeyboardChange called ExpectForgedMessage, which not only checks an upcoming forged event (such as the ControlLeft up forged by an AltGr up), and also holds succeeding KeyStateChange until the forged event occurs. This is necessary to correctly test the AltGr behaviors.

This PR also allows mocking all kinds of MapVirtualKey instead of just VK_TO_CHAR, although it turns out it's not used in tests.

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.

If you need help, consider asking for advice on the #hackers-new channel on Discord.