refactor(theme-ui): add typescript to the root package

[hasparus]

Part of #668.

Useful for #705.

Emitted .d.ts

/// <reference types="@theme-ui/core/dist/react-jsx" />
export { jsx, Context, merge, useThemeUI, ContextValue, IntrinsicSxElements, SxProps, SxStyleProp, } from '@theme-ui/core';
export { useColorMode, InitializeColorMode } from '@theme-ui/color-modes';
export { Styled, components } from '@theme-ui/mdx';
export { ThemeProvider } from '@theme-ui/theme-provider';
export * from '@theme-ui/components';
export { css, get, ColorMode, CSSObject, CSSProperties, CSSPseudoSelectorProps, ResponsiveStyleValue, SystemCssProperties, SystemStyleObject, Theme, TLengthStyledSystem, UseThemeFunction, } from '@theme-ui/css';
export declare const BaseStyles: (props: Record<string, unknown>) => import("react").DetailedReactHTMLElement<{
    sx: {
        fontFamily: string;
        lineHeight: string;
        fontWeight: string;
        variant: string;
    };
}, HTMLElement>;

[mxstbr]

Awesome, that looks great! Thank you @hasparus!