Add MessageChannel#sendTypingContinuously methods to send typing indicator during a try-with-resources block

[Vampire]

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Changes

• Internal code
• Library interface (affecting end-user code)
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• Other: _____

Closes Issue: NaN

Description

This PR makes this possible:

try (var _ = messageChannel.sendTypingContinuously()) {
    // do lengthy stuff
}
sendReply();