Support passthrough dynamic import, or basic code doesn't work

Please at least support a basic passthrough for dynamic import for Closure Compiler. In the current version, any presence of dynamic import at all causes a compilation failure. This causes even basic use cases of dynamic import to cause the entire compilation to fail.

Example in.js:

```js
(async () =>
{
await import("https://cdn.example.com/library.js");
externalLibrary.someFunc();
})();
```

Command line:

`java -jar ./closure-compiler.jar --js in.js --js_output_file out.js --compilation_level SIMPLE --formatting PRETTY_PRINT --language_in ECMASCRIPT_2020 --language_out ECMASCRIPT_2020`

Observed result:
```
in.js:4:7: WARNING - [JSC_PARSE_ERROR] Parse error. This language feature is not currently supported by the compiler: Dynamic module import
4| await import("https://cdn.example.com/library.js");
^

ERROR - [JSC_FEATURES_NOT_SUPPORTED_BY_PASS] Attempted to run pass "markUntranspilableFeaturesAsRemoved" on input with features it does not support. Running pass anyway.
Unsupported features: [Dynamic module import]

1 error(s), 1 warning(s)
```
No output file is generated.

Expected result:

Just output the file with all transforms, but passthrough `import(XYZ)` -> `import(XYZ)`

This is blocking us using JavaScript modules at all in our code. We want to insert a dynamic import, but then Closure Compiler will refuse to process any of our code at all.