add `promise` external
add `import` external (uses import())
add `output.importFunctionName` option to change the `import()` function name
allow for inline external type when using arrays
fix some typings
fix namespace object behavior when using system external and accessing nested property
fix interop behavior for async dynamic modules
fix a bug with prefetching initial chunks
InitFragments have get[End]Content methods which get a sourceContext
RuntimeModules have implicit chunk and compilation
changed default to `output.ecmaVersion: 2015`