Block splitting - Handle the case when the split occurs at the trailing or leading edge

[daniloercoli]

This PR adds handles the case when the split occurs at the trailing or leading edge of the field.

Testing:

1. Put the cursor at the end of a para block and hit enter. It should add an empty block after the current.
2. Put the cursor at the end of a para block and hit Enter. It should add a block after the current with all the content. Current block will be blank after.
3. Put the cursor in the middle of a block, and hit Enter.

Note: there are some known issues with Heading blocks and validation. Will address that in another PR.

Companion PR:

• GB side: Android Fix - cursor position - Return the exact location of the cursor in the editor react-native-aztec#61
• AztecReactNative: ReactNative - Block splitting - Handle the case when the split occurs at the trailing or leading edge WordPress/gutenberg#10737

[diegoreymendez]

Works as described. Awesome work!

Two notes:

• As mentioned by @daniloercoli, the heading blocks have some validation issues when switching to HTML and back.
• The heading blocks don't work very well when splitting.
• Blocks are not properly resized when split.

[diegoreymendez]

Related to #167