* Early draft of SplitButton component.
* Changelog for SplitButton component.
* Rename SplitButton to SplitButtonDropdown, apply styles to address toggle button location/size, take primary button from children.
* Address button variant styles
* Rename 'Default' SplitButtonDropdown story to 'Secondary' for clarity.
* Add a SplitButtonDropdown story where the component is disabled.
* README for SplitButtonDropdown component.
* Add disabled prop to SplitButtonDropdown README.
* Add a SplitButtonDropdown story where some but not all actions are disabled.
* Restore PrimaryDisabled story for SplitButtonDropdown.
* Reorganize SplitButtonDropdown properties inheritance to allow both disabled and variant to apply to both the main and toggle buttons.
* Rename SplitButtonDropdown to SplitDropdown.
* Hardcode menuIcon and menuIconExpanded in SplitDropdown.
* Fix outdated stylesheet import for SplitDropdown.
* Update SplitDropdown changelog to new component name.
* Update variable names that reference Actions to Items.
* Hardcode expanded/collapsed toggle button icons without extraneous const declarations.
* Use classNames() helper fn for className concatenation.
* Increase width of SplitDropdown toggle button.
* Add a specific className to the root ButtonGroup element of SplitDropdown.
* Simple rendering test for SplitDropdown.
* For consistency, move Button text out of props in SplitDropdown README and story.
* SplitDropdown does not render a toggle button if there is nothing to put in the menu.
* Fix variant prop type declaration in SplitDropdown.
* Improve SplitDropdown test: remove snapshot, add toggle click.
Co-authored-by: Joshua Flowers <joshuatf@gmail.com>