clang-format: [JS] support null operators.

Summary:
JavaScript / TypeScript is adding two new operators: the null
propagating operator `?.` and the nullish coalescing operator `??`.

    const x = foo ?? 'default';
    const z = foo?.bar?.baz;

This change adds support to lex and format both.

Reviewers: krasimir

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69971