fix(button & dropdownitem): reverts return type of Button & DropdownItem to JSX.Element

In themesberg#1244, the return type of Button and DropdownItem was changed to ReactNode. This is not an
acceptable return type for functional components. The correct return type is JSX.Element. This
change fixes the regression from that PR (introduced in v v0.7.3 A Also fixes  themesberg#962.

fix themesberg#962

Author: rnicholus

src/components/Button/Button.tsx

@@ -90,7 +90,7 @@ export type ButtonProps<T extends ElementType = 'button'> = PolymorphicComponent
 
 type ButtonComponentType = (<C extends React.ElementType = 'button'>(
   props: ButtonProps<C>,
-) => React.ReactNode | null) & { displayName?: string };
+) => JSX.Element | null) & { displayName?: string };
 
 const ButtonComponentFn: ButtonComponentType = forwardRef(
   <T extends ElementType = 'button'>(

src/components/Dropdown/DropdownItem.tsx

@@ -27,7 +27,7 @@ export type DropdownItemProps<T extends ElementType = 'button'> = PolymorphicCom
 
 type DropdownItemComponentType = (<C extends React.ElementType = 'button'>(
   props: DropdownItemProps<C>,
-) => React.ReactNode | null) & { displayName?: string };
+) => JSX.Element | null) & { displayName?: string };
 
 export const DropdownItem: DropdownItemComponentType = forwardRef(
   <T extends ElementType = 'button'>(