Fix broken Chinese IME when deleting composition

[leonMSFT]

Summary of the Pull Request

This PR fixes an out of bounds access when deleting composition during Chinese IME. What's happening is that we're receiving a CompositionCompleted before receiving the TextUpdate to tell us to delete the last character in the composition. This creates two problems for us:

1. The final character gets sent to the Terminal when it should have been deleted.
2. _activeTextStart gets set to _inputBuffer.length() when sending the character to the terminal, so when the TextUpdate comes right after the CompositionCompleted event, _activeTextStart is out of sync.

This PR fixes the second issue, by updating _activeTextStart during a TextUpdate in case we run into this issue.
The first issue is trickier to resolve since we assume that if the text server api tells us a composition is completed, we should send what we have. It'll be tracked here: #5110.
At the very least, this PR will let users continue to type in Chinese IME without it breaking, but it will still be annoying to see the first letter of your composition reappear after deleting it.

PR Checklist

• Closes IME fails very frequently in the v0.10 release #5054
• CLA signed. If not, go over here and sign the CLA
• Tests added/passed

Validation Steps Performed

Play around with Chinese IME deleting and composing, and play around with Korean and Japanese IME to see that it still works as expected.

[ghost]

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[DHowett-MSFT]

@msftbot merge this in 8 minutes

[ghost]

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[ghost]

🎉Windows Terminal Preview v0.11.1121.0 has been released which incorporates this pull request.:tada:

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