[CC] Improve IME support

[kswenson]

Is this adding or improving a feature or fixing a bug?

Fixing IME bugs

What's the new behavior?

1. Caret stays at correct location during IME input.
2. insertFromComposition event handled properly.

How does this change work?

Cobbles together proposed fixes from the following issues/pull requests/comments:

• https://github.com/ianstormtaylor/slate/pull/2415/files#r232777986
• https://github.com/ianstormtaylor/slate/pull/2454/files#diff-021b2a72cf52cd5e8214d0a035b3ebc9R351
• https://github.com/ianstormtaylor/slate/pull/3126/files#diff-021b2a72cf52cd5e8214d0a035b3ebc9R488
• fix editing with IMEs ianstormtaylor/slate#2368 (comment)
• https://github.com/ianstormtaylor/slate/pull/3126/files#diff-66ff60bf0ae1c9b0d0ef4497bbe44cebR111

Have you checked that...?

• The new code matches the existing patterns and styles.
• The tests pass with yarn test.
• The linter passes with yarn lint. (Fix errors with yarn fix.)
• The relevant examples still work. (Run examples with yarn start.)

Does this fix any issues or need any specific reviewers?

Fixes: #
Reviewers: @