Calling `load()` on a lazy loaded element doesn't work

[latentflip]

I've discovered a bug where calling load() manually on a lazy loaded breaks the element. It triggers the request, but then when the element becomes visible the content isn't replaced. Example below from the examples page:

bug.mov

[latentflip]

There appear to be two separate bugs at play here:

1. The load() function un-observes the intersection observer, which means when the element becomes visible it will not trigger content replacement:

   include-fragment-element/src/index.ts

   Line 149 in 5785cf3

   observer.unobserve(this)

2. Removing that line will fix the bug, but the preloading doesn't actually work, a second request for the content will be made when the element becomes visible. That's because the load() function doesn't actually store the result anywhere.