Support dynamic import()

[Rich-Harris]

Just started playing around with Svelte+webpack (probably something I should have tried to get my head round a long time ago), and immediately ran into an annoyance — because acorn doesn't support dynamic import(...), neither does Svelte.

It'd be nice if that worked out of the box. There's a plugin for it, so it should be relatively straightforward.

Open question whether any other ESnext stuff should be supported, but I think import(...) passes whatever threshold we'd set.