import assertions with swc transpiler

swc transpiler needs to be passed necessary flags to preserve import assertions. Currently, it omits them from emit, which is wrong.

Must pass two flags:

```typescript
{
 jsc: {
 parser: {
 importAssertions: true
 },
 experimental: {
 keepImportAssertions: true
 }
 }
}
```

Reported in #1810.

Will be fixed by #1802 