Fix unicode offset

[davidhoeller]

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

This is basically PR #2680 in the current version of slate (Fixes #2635, fixes #2632)
Thanks @adjourn for starting this.
Plus, fixes a bug where the cursor could not be moved beyond block with arrow keys.

What's the new behavior?

As explained in #2680 .
The emoji example has been extended to insert emojis as plain text (vs inline nodes) to demonstrate the new behaviour.

How does this change work?

See #2680
Updated emoji example, see here:

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 prettier.)
• The relevant examples still work. (Run examples with yarn watch.)

Does this fix any issues or need any specific reviewers?

Fixes: #2635, #2632
Reviewers: @adjourn @ianstormtaylor @kwokhuen

[ianstormtaylor]

Thanks @davidhoeller for update this! And huge thank you to @adjourn for taking the time to fix this in the first place.

[sammylupt]

Thank you so much!