allow theme borderRradius to take in string

[dev-seahouse]

Currently theme object only takes in number,

// react-select/src/types.ts
export interface Theme {
  borderRadius: number;
  colors: Colors;
  spacing: ThemeSpacing;
}

This means that such valid use case results in typescript error:

<Select  theme={theme=> ({
   ...theme,
   borderRadius: `var(--myBorderRadius)`
}) }/>