Better TS enum support

[shilman]

Reporting from storybookjs/storybook#24165

Currently TS enums analysis is lossy:

enum ButtonType {
  Button = 'button',
  Reset = 'reset',
  Submit = 'submit',
}

type ButtonTestProps = {
  type?: ButtonType;
};

export const ButtonTest = (props: ButtonTestProps) => {
  return <button {...props}>Test</button>;
};

Produces:

          "tsType": {
            "name": "ButtonType"
          },

Compare this to a union:

type ButtonType = "button" | "reset" | "submit";

Which produces:

          "tsType": {
            "name": "union",
            "raw": "\"button\" | \"reset\" | \"submit\"",
            "elements": [
              {
                "name": "literal",
                "value": "\"button\""
              },
              {
                "name": "literal",
                "value": "\"reset\""
              },
              {
                "name": "literal",
                "value": "\"submit\""
              }
            ]
          },

It would be fantastic if react-docgen could produce something similar for enums!