Support Nested Lists

[jolanglinais]

Is your feature request related to a problem? Please describe.
The editor should support nested lists with a keyboard handle for SHIFT + ENTER and toolbar buttons for indent and outdent

Describe the solution you'd like
Design from @Michael-Grover, this should wrap a list in a further list if it is a descendent of a list

Describe alternatives you've considered
Further work can be done on indenting paragraphs in the future

Additional context
Lists in the schema should support other lists

• Issue Slate Losing Lists markdown-editor#192
• PR fix(src): handle new form of list item as paragraph - I192 markdown-editor#194

Account for the commented out underline code

• Issue Remove Underline Button from Toolbar markdown-editor#97
• PR chore(*): comment out underline-related code - I97 markdown-editor#99