Refact constants into separate type

[heaths]

I think this could be folded into defaultValue as a new Constant type (might be useful elsewhere).

/** Constant defines a Rust const declaration */
export interface Constant {
  kind: 'constant';

  /** the name of the const */
  name: string;

  /** the value of the const */
  value: Literal;

  /** indicates the visibility of the const */
  visibility: Visibility;
}

Then we can update the definition accordingly.

  defaultValue?: Constant | string;

Originally posted by @jhendrixMSFT in #887 (comment)