`src` should be an optional prop for Plausible

[nevanscott]

I'm not sure I've approached this the right way, but my type checker was throwing an error if I used the <Plausible> component without a src. Since there's a default, I believe this should be an optional prop, and it looks like the previous approach of defining the type as src: string | undefined was not cluing the type checker in on this. Other astro plugins I looked at were using something like the convention I used here, src?: string, so I am proposing that as a change.

I believe that this is the more correct notation for this.

[Destiner]

Thanks @nevanscott, makes sense to me!