Revert "native_viewer is now an opt-in feature of the rerun library"

[emilk]

Reverts #2064

Turns out cargo r -p rerun will no longer work, you need to run cargo r -p rerun --features native_viewer. Same for cargo install rerun for our users. This sucks.

> cargo r -p rerun
error: target `rerun` in package `rerun` requires the features: `native_viewer`
Consider enabling them by passing, e.g., `--features="native_viewer"`

Re-opens #1997