What does this PR do?

This PR adds an extend to the theme for the "listbox" container directly surrounding the options. Open to naming suggestions, but things like "option.container" etc were already taken and apply to different things. "Listbox" is because this is the container with role="listbox".

Had to pass through selectMultiple as a prop on OptionsContainer so that the extend could differentiate between desired styling for Select vs SelectMultiple

Where should the reviewer start?

src/js/components/Select/StyledSelect.js

What testing has been done on this PR?

Locally. Also added jest tests.

How should this be manually tested?

theme: {
   select: {
      listbox: {
          extend: `padding: 24px;`,
      },
   },
  selectMultiple: {
      listbox: {
          extend: `padding: 12px;`,
      },
   },
}

Do Jest tests follow these best practices?

• screen is used for querying.
• The correct query is used. (Refer to this list of queries)
• asFragment() is used for snapshot testing.

Any background context you want to provide?

What are the relevant issues?

Screenshots (if appropriate)

Do the grommet docs need to be updated?

Yes.

Should this PR be mentioned in the release notes?

Yes.

Is this change backwards compatible or is it a breaking change?

Backwards compatible.