import assertions with swc transpiler

[cspotcode]

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:

{
  jsc: {
    parser: {
      importAssertions: true
    },
    experimental: {
      keepImportAssertions: true
    }
  }
}

Reported in #1810.

Will be fixed by #1802