DefaultParams type doesn't count with optional parameters

[Poky85]

Wouter export types:

export interface DefaultParams {
  [paramName: string]: string;
}
export type Params<T extends DefaultParams = DefaultParams> = T;

Let's use dynamic segment in the path, eg:

<Route path="/:foo/:bar?">
  {(params) => {
    
  }}
</Route>;

...and match the route against path /hello. Then matcher returns params as following: { foo: hello, bar: undefined }. But those params doesn't comply with DefaultParams definition. So I think it should be:

export interface DefaultParams {
  [paramName: string]: string | undefined;
}

[molefrog]

Hey @itsMapleLeaf @cbbfcd could you take a look at the proposed change?

[cbbfcd]

@Poky85 may be resolved by #211 .

and @itsMapleLeaf, It looks like we need to add some test cases 😂