Support TypeScript 4.4 exactOptionalPropertyTypes

[aaronadamsCA]

Is your feature request related to a problem? Please describe.
Typescript 4.4 introduced a new feature, exactOptionalPropertyTypes. The default value is false but the recommended value is true.

Describe the solution you'd like
Theme UI needs some type improvements to work correctly with this setting enabled.

Additional context
Here is an example of an error that occurs only when this setting is enabled:

import type { BoxProps } from "@theme-ui/components";
import { Box } from "@theme-ui/components";

export const Example = (props: BoxProps): JSX.Element => <Box {...props} />;

The error appears on Box:

Type '{ slot?: string | undefined; style?: CSSProperties | undefined; title?: string | undefined; key?: Key | null | undefined; defaultChecked?: boolean | undefined; ... 285 more ...; opacity?: ResponsiveValue<...> | undefined; }' is not assignable to type 'BoxOwnProps'.
  Types of property 'color' are incompatible.
    Type 'string | undefined' is not assignable to type 'ResponsiveValue<never, Required<Theme<TLengthStyledSystem>>> | undefined'.
      Type 'string' is not assignable to type 'ResponsiveValue<never, Required<Theme<TLengthStyledSystem>>> | undefined'. ts(2322)

I believe the issue may be upstream in @types/styled-system, but I don't understand how to diagnose it well enough to report it. I'm hoping somebody with more TypeScript knowledge could figure out how to get this working.