Using variable size IME events

[jimon]

Description

Continuation of work from #1120

Unity historically was using fixed size IME event type with 64 characters, which is not sufficient in some cases, like complex emojis. Some years ago a new IME event type was added that instead contains a variable size array of characters that solves the problem above.

Native code reports both events at a same time on all platforms where IME is implemented.

Changes made

• Adding new event type.
• Using it for IME composition events.
• Modifying IMECompositionString to be a small string optimization container, where below 64 characters it will not allocate, but above will do a managed allocation.
• Slightly breaking the API as IMECompositionString cannot be used in the old event anymore due to adding a new field there.

[unity-cla-assistant]

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:x: Dmytro Ivanov
:x: StayTalm

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