fix(theme-ui): add missing FlexProps type (#42207)

* fix(theme-ui): add missing FlexProps type

* chore(theme-ui): add self as an author

types/theme-ui__components/index.d.ts

@@ -1,6 +1,7 @@
 // Type definitions for @theme-ui/components 0.2
 // Project: https://github.com/system-ui/theme-ui
 // Definitions by: Piotr Monwid-Olechnowicz <https://github.com/hasparus>
+//                 Kristóf Poduszló <https://github.com/kripod>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 // TypeScript Version: 3.5
 
@@ -34,6 +35,7 @@ export interface BoxProps extends Assign<React.ComponentProps<'div'>, BoxOwnProp
 export const Box: StyledComponent<React.ComponentProps<'div'>, BoxOwnProps, {}>;
 
 export type FlexStyleProps = BoxOwnProps;
+export type FlexProps = BoxProps;
 /**
  * Use the Flex component to create flexbox layouts.
  * @see https://theme-ui.com/components/flex