Show new block indicator in default appender too

[hypest]

Description

Showing the "ADD BLOCK HERE" block insertion point indicator when the block list is fully empty too. Also, at the end of the list when not empty

Gutenberg-mobile PR: wordpress-mobile/gutenberg-mobile#1542

How has this been tested?

Using the gutenberg-mobile PR.

Types of changes

1. Revised the default mobile block appender to show the new-block insertion point indicator instead of the default paragraph block, when the related flag is true and the block list is empty.
2. Introduced a method to check if the indicator at the end of the block should appear, when the insertion point is at the end of the list.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.

[mchowning]

Looks good and works well! I really like the explanatory comments you added. 🎉