Try: Improve visual style and usability of HTML mode

[iamthomasbishop]

There a few addressable issues with HTML Mode, and a few bonus ones that could be worth exploring in the future.

Current state:

Low-Hanging Fruit

• Readability: Font size is too small. It should be about the same font size as Visual More.
• Readability: There is no syntax highlighting in HTML Mode.
• There isn't a very large distinction between Visual and HTML Mode (this includes but not limited to the editor canvas).
• Post Title could be styled the same as it is in Visual.
• [bug] Pasting external HMTL into editor creates an Unsupported block. Should this be wrapped in an HTML block?

Bonus points if we can address

• Writing flow is clunky bc of the nature of writing code with a mobile keyboard, esp on phone. We could utilize the Quick Toolbar space or voice commands to provide useful shortcuts.
• There are many other apps that are designed for writing code, such as Coda. Explore sharing/editing HTML contents in external editors.

Coda for iOS, for reference:

[SergioEstevao]

I don't see an easy way to implement this in RN only, with a cross platform solution so
in order to address all these issues we will need to use/create a native component for iOS and Android.

On iOS we can reuse the component that we did for Aztec, I'm not aware what is the situation on Android, @hypest do you know?

[hypest]

Aztec Android also has a "html mode" editor and we should investigate integrating it.

[hypest]

Syntax highlighting also asked in #3999833-zen.