Use using enum for JsonOptions (C++20)

This CL updates the JsonWriter class to use the `using enum` declaration
for JsonOptions, which is a C++20 feature. This removes the need for the
previous workaround that manually defined static constexpr variables for
each enum value.

Change-Id: Id9a567f5e52cefca217cd14c03ceb5a53143aa18
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6072783
Reviewed-by: Francois Pierre Doray <fdoray@chromium.org>
Commit-Queue: Alex Attar <aattar@google.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1392849}
NOKEYCHECK=True
GitOrigin-RevId: 88cdc6d086bdcf6a6fb1f14de4af7a0f72356706
2 files changed