Fix multiline dynamic import comments

[noahsug]

Fixes #4617

Trying to keep dynamic imports as a single unit (#2288) and not allowing comments to be attached to dynamic imports (#2748) was causing weird behavior when comments existed on a different line than the argument. This often comes up when using webpack's magic comments.

I also removed tests/import_then/ in favor of the existing tests/dynamic_import/.

// Input
import(
  /* Hello */
  'something'
  /* Hello */
)
import(
  'myreallylongdynamicallyloadedmodulenamemyreallylongdynamicallyloadedmodulename'
)

// Output (Prettier stable)
import(/* Hello */
"something");
/* Hello */
import('myreallylongdynamicallyloadedmodulenamemyreallylongdynamicallyloadedmodulename');


// Output (Prettier master)
import(
  /* Hello */
  'something'
  /* Hello */
)
import(
  'myreallylongdynamicallyloadedmodulenamemyreallylongdynamicallyloadedmodulename'
);

Note: this is still an issue for require and define, but I imagine multiline comments are much less popular in those methods.

• I’ve added tests to confirm my change works.
• (If the change is user-facing) I’ve added my changes to the CHANGELOG.unreleased.md file following the template.
• I’ve read the contributing guidelines.

✨Try the playground for this PR✨

[noahsug]

I ran this on Airbnb's codebase as an integration test and confirmed that everything looks good ✨

[vjeux]

Thanks a lot for fixing this and the great test coverage + docs! I’ve also seen this be really annoying for webpack comments.