bug(rich-text) down-arrow in code block exits the code block even when more lines in the code block below cursor

[yellis]

Describe the bug
Pressing the down arrow key in a code blocks with multiple lines moves out of the code block, instead of down to the next line in the code block.

To Reproduce

1. RT mode
2. Add text with multiple lines, make into code block
3. Cursor on the top line
4. Down arrow on keyboard
5. Observe: cursor skips remaining lines in the code block, exits code block

Expected behavior
If additional lines below cursor in the code block, down arrow should move to next line. Down arrow should only exit code block when cursor is on the last line in the code block.

Screenshots

In demo, user pressed: down up up

Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context
Add any other context about the problem here.

[KyleMit]

Potentially related to #199 ?