Create new snippet in between other snippets

## Description
This is the initial design and set up for the functionality of adding new snippets in between existing one. The feature has been dubbed as an "adder-bubble" which will darken in color and expand height when a user hovers over it.

### Testing Considerations
*[] The design works well with existing branding
*[] Hover expands height without breaking
*[] The adder bubble doesn't break for width changes
*[] The transition works on chrome, firefox, safari