track whether block is already outroing

[Rich-Harris]

General sketch of a solution for #3001 (and maybe #2402?). It should probably apply to all if blocks, and maybe non-if-blocks as well, but I started by fixing the narrow repro.

I can't shake the feeling that this is doubling up somewhere, e.g. with the current variable.

Couldn't get a runtime test to fail, oddly, so put the test in the js directory instead.

[Rich-Harris]

So, this affects any block that Svelte tries to remove while it's already being removed: https://svelte.dev/repl/3fe3f164eab4445c9606240080442e59?version=3.5.1. We could add similar if (!outroing_this_thing) guards around each of these blocks, but it's starting to feel very brittle and hacky. Would really prefer if that logic was centralised somehow. For now, staring at some generated code and hoping for inspiration to strike.

[Rich-Harris]

closing in favour of #3046