PropItem.defaultValue is typed as `any` instead of a more narrow type

[lukasnys]

The defaultValue field on PropItem is typed as any, but at runtime it is always either null or { value: ... }. This means TypeScript gives no warning when consumers access prop.defaultValue.value without a null check.

Suggested fix

// Before
export  interface PropItem {
  ...
  defaultValue: any;
  ...
}

// After
export  interface PropItem {
  ...
  defaultValue: { value: any } | null;
  ...
}