What does this PR do?

Adds support for custom padding for icon-only buttons (buttons that have icon and no label). Having the ability to specify icon only padding is desirable to create uniform dimension icon only buttons.

Additionally, it is desirable to be able to manage icon size from the theme and vary the icon's size depending on what size button it is contained in.

This PR adds:

• theme.button.icon.size (allows caller to specify what size the icon should render as when contained in a small, medium, or large button

  icon?: {
    size?: {
      small?: string;
      medium?: string;
      large?: string;
    };
  };

• theme.button.size[size].iconOnly.pad which allows the caller to specify pad for iconOnly pad across button sizes.

Where should the reviewer start?

base.js

What testing has been done on this PR?

Tested in local story:

How should this be manually tested?

Do Jest tests follow these best practices?

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

Any background context you want to provide?

What are the relevant issues?

Related to: grommet/hpe-design-system#3117

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.