[Gutenberg] Account for the end of buffer marker

[chipsnyder]

Fix

wordpress-mobile/gutenberg-mobile#1663
wordpress-mobile/gutenberg-mobile#1524

When deleting text in the Heading block the end of buffer marker prevents the handleBackspaceAndEnter event from firing resulting in weird behavior like the placeholder not showing up. This also affects formatting updates.

To resolve this I now account for that character in emptyEditTextBackspaceDetector check these changes will allow the

Heading block

1. Open the Gutenberg Editor
2. Create a Heading block containing some text
3. Press backspace until all of the text has been deleted
   Expect to see the placeholder text return after the last character is deleted

Before

After

Paragraph block

1. Open the Gutenberg Editor
2. Create a Paragraph block containing some text
3. Select the full text and apply some styling like bold
4. Press backspace until all of the text has been deleted
   Expect to see the placeholder text return after the last character is deleted

Before

After

Review

@khaykov I think you were the last one to work in this area if you have time to review if not just let me know and I can ask someone working on Gutenberg to review.

Make sure strings will be translated:

• If there are new strings that have to be translated, I have added them to the client's strings.xml as a part of the integration PR.

[khaykov]

I confirmed the issue and that the fix works! Thanks for the fix 👍