feat: add separator block

[IILQ12]

There is no option to insert separators, such as lines, in the message.

Example:

[dominikagoral]

I created a branch with the mj-divider block that does just this, but it doesn't get rendered in Outlook. This may get fixed in a future update of Outlook or MJML, but in the meantime I would recommend using underscores for this purpose.