clang-format: [JS] sort import aliases.

Users can define aliases for long symbols using import aliases:

    import X = A.B.C;

Previously, these were unhandled and would terminate import sorting.
With this change, aliases sort as their own group, coming last after all
other imports.

Aliases are not sorted within their group, as they may reference each
other, so order is significant.

Revision URI: https://reviews.llvm.org/D118361