feat(split-button): add ts type definitions

src/components/split-button/index.d.ts

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+export { default } from "./split-button";

src/components/split-button/split-button.d.ts

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+import * as React from "react";
+import * as OptionsHelper from "../../utils/helpers/options-helper/options-helper";
+
+export interface SplitButtonProps {
+  /** Set align of the rendered content */
+  align?: OptionsHelper.AlignBinaryType;
+  /** Button type: "primary" | "secondary" for legacy theme */
+  as?: OptionsHelper.ThemesBinary;
+  /** Button type: "primary" | "secondary" */
+  buttonType?: OptionsHelper.ThemesBinary;
+  /** The additional button to display. */
+  children: React.ReactNode;
+  /** A custom value for the data-element attribute */
+  "data-element"?: string;
+  /** A custom value for the data-role attribute */
+  "data-role"?: string;
+  /** Gives the button a disabled state. */
+  disabled?: boolean;
+  /** Defines an Icon position within the button: "before" | "after" */
+  iconPosition?: "before" | "after";
+  /** The size of the buttons in the SplitButton. */
+  size?: OptionsHelper.SizesRestricted;
+  /** The text to be displayed in the SplitButton. */
+  text: string;
+}
+
+declare class SplitButton extends React.Component<SplitButtonProps> {}
+
+export default SplitButton;