Add tests

[JohnnyFun]

I may get to this at some point, but wanted to note it in case you're already working on it.

Some cases (not exhaustive):

• basic scenario: <Loadable component={Component} />
• with props: <Loadable component={Component} some="value" />
• with page loader: <Loadable loader={() => getComponentDynamically()} />
• with registered loader: <Loadable loader={register({ loader, resolve })} />
• with default and loading slot: <Loadable ...><div slot="loading">loading...</div>...</Loadable>
• others...but those ^ would be a good start

Could use cypress and cypress-svelte-unit-test. Or the test library that svelte itself uses. Or whatever the implementer's heart desires, so long as it asserts what needs to be asserted :)