This PR fixes issues that have been impacting the Vietnamese IME, UniKeyNT Telex Mode, the windows part of flutter/flutter#82673.

This IME adds or removes diacritics of the entire word when the last letter of the word is typed by forging Backspace keys and new characters. For example,

To get ào, you type aof. The detailed messages are:

• Tap KeyA, messages are KeyDown(KeyA), Char(a), KeyUp(KeyA).
• Tap KeyO, messages are KeyDown(KeyO), Char(o), KeyUp(KeyO).
• Press KeyF, messages are
  • KeyDown(Backspace), Char(Backspace), KeyUp(Backspace).
  • KeyDown(Backspace), Char(Backspace), KeyUp(Backspace).
  • KeyDown(VK_PACKET), Char(à), KeyUp(VK_PACKET).
  • KeyDown(VK_PACKET), Char(o), KeyUp(VK_PACKET).
• Release KeyF, messages are KeyUp(KeyF).

The first change of this PR is to process the VK_PACKET session, which is easy because we can just ignore the VK_PACKET messages and take the body char messages as standalone char messages.

The second change, which is way harder, is to fix a racing condition. Both the engine and the framework maintains a state of the text input field currently being edited. Sometimes the framework sends an updated state to the engine, sometimes the other way around. More specifically,

1. When the KeyDown(Backspace) events are received by the framework, the framework processes it with the shortcut system and decides that it affects the text state, deletes the last character of the text, and sends the updated state to the engine.
2. When the printable Char messages are being processed by the engine, the engine decides that it should update the engine's text state immediately, and sends the updated state to the framework.

However, since all the messages triggered by the KeyF press are so tightly queued, that the first text state update above will usually, but not always, arrive after the second text state update is done locally. The symptom is that after pressing f, the resulting text can be either ào, ao, o, or some other random values.

The best way to fix it might be to make EditableText wait for the text state update to finish (https://github.com/flutter/flutter/blob/master/packages/flutter/lib/src/widgets/editable_text.dart#L2171). However this might be too big of a change.

The fix proposed in this PR is to make unprintable chars (such as Backspace) take up a placeholder char in the char queue, and remove this char only when the corresponding keydown message has arrived. Since the platform messenger maintains message order, and that TextInput._instance._setEditingState(value) synchronously pushes an message to the messenger, it's guaranteed that the text state update arrives at the engine before the response of the key down event. However, this fix does not prevent the case if a key without any characters would like to change the text state.

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.