Support top level await

[dsherret]

Describe the feature

Would be nice to now support top level await:

async function f() {
}
await f();

I believe this would be a feature that isn't compiled and is only supported when targeting es2017.

https://devblogs.microsoft.com/typescript/announcing-typescript-3-8-beta/#top-level-await

[dsherret]

@kdy1 I think there is a slight bug in that code. Looks like it shifted the class declaration position. It should be 0: https://github.com/swc-project/swc/pull/627/files#diff-8216cd047b4ca3e65cb89734b4697652

See here: https://github.com/kdy1/swc/blob/24af0df8109a7eeea0bef7baa138e38200171ed4/ecmascript/parser/tests/typescript/class/decorators/input.ts

[dsherret]

I'll submit a fix.

[swc-bot]

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