clang-format: [JS] merge import lines.

Multiple lines importing from the same URL can be merged:

    import {X} from 'a';
    import {Y} from 'a';

Merge to:

    import {X, Y} from 'a';

This change implements this merge operation. It takes care not to merge in
various corner case situations (default imports, star imports).

Differential Revision: https://reviews.llvm.org/D100466